Added a files endpoint, corrected a documents bug in answers requests, added error messages to API error handling (#6)

Co-authored-by: eyelevelai <33876565+eyelevelai@users.noreply.github.com>

error.go

@@ -0,0 +1,10 @@
+package gogpt
+
+type ErrorResponse struct {
+	Error *struct {
+		Code    *int    `json:"code,omitempty"`
+		Message string  `json:"message"`
+		Param   *string `json:"param,omitempty"`
+		Type    string  `json:"type"`
+	} `json:"error,omitempty"`
+}