actual un-checked error fixes

atproto/lexicon/cmd/lextool/net.go

···
 	}
 
 	body, err := data.UnmarshalJSON(respBytes)
+	if err != nil {
+		return err
+	}
 	record, ok := body["value"].(map[string]any)
 	if !ok {
 		return fmt.Errorf("fetched record was not an object")

atproto/repo/mst/node_insert.go

···
 	}
 
 	// include "covering" proof for this operation
-	proveMutation(n, key)
+	if err := proveMutation(n, key); err != nil {
+		return nil, nil, err
+	}
 
 	if !split {
 		// TODO: is this really necessary? or can we just slices.Insert beyond the end of a slice?

atproto/repo/mst/node_remove.go

···
 	}
 
 	// marks adjacent child nodes dirty to include as "proof"
-	proveMutation(n, key)
+	if err := proveMutation(n, key); err != nil {
+		return nil, nil, err
+	}
 
 	// check if top of node is now just a pointer
 	if top {