cli: show executable name in error message, include underlying error details

Since this is the error to spawn (or wait) process, command arguments aren't
important. Let's make that clear by not showing full command string.

#2614

cli/src/cli_util.rs

···
         .config()
         .get("ui.editor")
         .map_err(|err| CommandError::ConfigError(format!("ui.editor: {err}")))?;
-    let exit_status = editor
-        .to_command()
-        .arg(edit_path)
-        .status()
-        .map_err(|_| user_error(format!("Failed to run editor '{editor}'")))?;
+    let exit_status = editor.to_command().arg(edit_path).status().map_err(|err| {
+        user_error(format!(
+            "Failed to run editor '{name}': {err}",
+            name = editor.split_name(),
+        ))
+    })?;
     if !exit_status.success() {
         return Err(user_error(format!(
             "Editor '{editor}' exited with an error"

cli/src/config.rs

···
 }
 
 impl CommandNameAndArgs {
+    /// Returns command name without arguments.
+    pub fn split_name(&self) -> Cow<str> {
+        let (name, _) = self.split_name_and_args();
+        name
+    }
+
     /// Returns command name and arguments.
     ///
     /// The command name may be an empty string (as well as each argument.)

cli/src/ui.rs

···
                     Err(e) => {
                         writeln!(
                             self.warning(),
-                            "Failed to spawn pager '{cmd}': {e}",
-                            cmd = self.pager_cmd,
+                            "Failed to spawn pager '{name}': {e}",
+                            name = self.pager_cmd.split_name(),
                         )
                         .ok();
                     }