gptel: Improve error extraction from JSON responses

Try a little harder to find error fields in JSON responses.
Eventually the error extractor can be a generic function with
API-specific methods, but it's not worth the complexity yet.

* gptel-curl.el (gptel-curl--stream-cleanup): Try looking inside
arrays for the error struct.
(gptel-curl--parse-response): Ditto.
* gptel.el (gptel--url-parse-response): Ditto.

Author: karthink

gptel-curl.el

@@ -241,7 +241,10 @@ PROCESS and _STATUS are process parameters."
                        (response (progn (goto-char header-size)
                                         (condition-case nil (gptel--json-read)
                                           (error 'json-read-error))))
-                       (error-data (plist-get response :error)))
+                       (error-data
+                        (cond ((plistp response) (plist-get response :error))
+                              ((arrayp response)
+                               (cl-some (lambda (el) (plist-get el :error)) response)))))
             (cond
              (error-data
               (plist-put info :error error-data))
@@ -460,8 +463,11 @@ PROC-INFO is a plist with contextual information."
                              ((not (string-blank-p resp))))
                     (string-trim resp))
                   http-status http-msg))
-           ((plist-get response :error)
-            (list nil http-status http-msg (plist-get response :error)))
+           ((and-let* ((error-data
+                        (cond ((plistp response) (plist-get response :error))
+                              ((arrayp response)
+                               (cl-some (lambda (el) (plist-get el :error)) response)))))
+              (list nil http-status http-msg error-data)))
            ((eq response 'json-read-error)
             (list nil http-status (concat "(" http-msg ") Malformed JSON in response.")
                   "Malformed JSON in response"))

gptel.el

@@ -3105,8 +3105,11 @@ See `gptel-curl--get-response' for its contents.")
                          ((not (string-blank-p resp))))
                 (string-trim resp))
               http-status http-msg))
-       ((plist-get response :error)
-        (list nil http-status http-msg (plist-get response :error)))
+       ((and-let* ((error-data
+                    (cond ((plistp response) (plist-get response :error))
+                          ((arrayp response)
+                           (cl-some (lambda (el) (plist-get el :error)) response)))))
+          (list nil http-status http-msg error-data)))
        ((eq response 'json-read-error)
         (list nil http-status (concat "(" http-msg ") Malformed JSON in response.") "json-read-error"))
        (t (list nil http-status (concat "(" http-msg ") Could not parse HTTP response.")