Add error formatting utilities to simplify error handling

Added string_of_error and pp_error functions to the Jmap.Api module
to provide a consistent, reusable way of formatting error messages.
This centralizes error formatting logic and reduces duplicated code
in client applications.

Updated the fastmail_list binary to use these functions, resulting
in cleaner, more maintainable code.

🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <[e-mail]>

bin/fastmail_list.ml

···
   let* mailboxes_result = Jmap_mail.get_mailboxes conn ~account_id in
   match mailboxes_result with
   | Error err ->
-      Printf.printf "Error getting mailboxes: %s\n" 
-        (match err with
-        | Jmap.Api.Connection_error msg -> "Connection error: " ^ msg
-        | Jmap.Api.HTTP_error (code, body) -> Printf.sprintf "HTTP error %d: %s" code body
-        | Jmap.Api.Parse_error msg -> "Parse error: " ^ msg
-        | Jmap.Api.Authentication_error -> "Authentication error");
+      Printf.printf "Error getting mailboxes: %s\n" (Api.string_of_error err);
       Lwt.return_unit
       
   | Ok mailboxes ->
···
           
           match emails_result with
           | Error err ->
-              Printf.printf "Error getting emails: %s\n" 
-                (match err with
-                | Jmap.Api.Connection_error msg -> "Connection error: " ^ msg
-                | Jmap.Api.HTTP_error (code, body) -> Printf.sprintf "HTTP error %d: %s" code body
-                | Jmap.Api.Parse_error msg -> "Parse error: " ^ msg
-                | Jmap.Api.Authentication_error -> "Authentication error");
+              Printf.printf "Error getting emails: %s\n" (Api.string_of_error err);
               Lwt.return_unit
               
           | Ok emails ->
···
       in
       match result with
       | Error err -> 
-          (match err with
-          | Api.Connection_error msg -> 
-              Printf.eprintf "Connection error: %s\n" msg
-          | Api.HTTP_error (code, body) -> 
-              Printf.eprintf "HTTP error %d: %s\n" code body
-          | Api.Parse_error msg -> 
-              Printf.eprintf "Parse error: %s\n" msg
-          | Api.Authentication_error -> 
-              Printf.eprintf "Authentication error. Check your API token.\n");
+          Printf.eprintf "%s\n" (Api.string_of_error err);
           Lwt.return 1
       | Ok conn ->
           (* Get the primary account ID *)
···
           let* mailboxes_result = get_mailboxes conn ~account_id:primary_account_id in
           match mailboxes_result with
           | Error err -> 
-              Printf.eprintf "Failed to get mailboxes: %s\n" 
-                (match err with
-                | Api.Connection_error msg -> "Connection error: " ^ msg
-                | Api.HTTP_error (code, body) -> Printf.sprintf "HTTP error %d: %s" code body
-                | Api.Parse_error msg -> "Parse error: " ^ msg
-                | Api.Authentication_error -> "Authentication error");
+              Printf.eprintf "Failed to get mailboxes: %s\n" (Api.string_of_error err);
               Lwt.return 1
           | Ok mailboxes ->
               (* If there's a mailbox list, just use the first one for this example *)
···
               in
               match emails_result with
               | Error err -> 
-                  Printf.eprintf "Failed to get emails: %s\n" 
-                    (match err with
-                    | Api.Connection_error msg -> "Connection error: " ^ msg
-                    | Api.HTTP_error (code, body) -> Printf.sprintf "HTTP error %d: %s" code body
-                    | Api.Parse_error msg -> "Parse error: " ^ msg
-                    | Api.Authentication_error -> "Authentication error");
+                  Printf.eprintf "Failed to get emails: %s\n" (Api.string_of_error err);
                   Lwt.return 1
               | Ok emails ->
                   (* Apply filters based on command line arguments *)

lib/jmap.ml

···
 
   (** Result type for API operations *)
   type 'a result = ('a, error) Stdlib.result
+  
+  (** Convert an error to a human-readable string *)
+  let string_of_error = function
+    | Connection_error msg -> "Connection error: " ^ msg
+    | HTTP_error (code, body) -> Printf.sprintf "HTTP error %d: %s" code body
+    | Parse_error msg -> "Parse error: " ^ msg
+    | Authentication_error -> "Authentication error"
+  
+  (** Pretty-print an error to a formatter *)
+  let pp_error ppf err =
+    Format.fprintf ppf "%s" (string_of_error err)
 
   (** Configuration for a JMAP API client *)
   type config = {

lib/jmap.mli

···
   (** Result type for API operations *)
   type 'a result = ('a, error) Stdlib.result
   
+  (** Convert an error to a human-readable string 
+      @param err The error to convert
+      @return A string representation of the error
+  *)
+  val string_of_error : error -> string
+  
+  (** Pretty-print an error to a formatter
+      @param ppf The formatter to print to
+      @param err The error to print
+  *)
+  val pp_error : Format.formatter -> error -> unit
+  
   (** Configuration for a JMAP API client as defined in RFC8620 Section 3.1
       @see <https://datatracker.ietf.org/doc/html/rfc8620#section-3.1>
   *)