stop logging errors for api error responses

From a cursory glance, when a real error occurs, we already log
something at the application layer, so there's no reason to echo every
api error response as an error log too. Errors should be actionable, but
these logs were not, e.g.:

> access denied to templates

or:

> category ID specified in input does not exist for user

server/api/api.go

@@ -178,7 +178,6 @@ func (a *API) userIsGuest(userID string) (bool, error) {
 // Response helpers
 
 func (a *API) errorResponse(w http.ResponseWriter, r *http.Request, err error) {
-	a.logger.Error(err.Error())
 	errorResponse := model.ErrorResponse{Error: err.Error()}
 
 	switch {
@@ -195,15 +194,16 @@ func (a *API) errorResponse(w http.ResponseWriter, r *http.Request, err error) {
 	case model.IsErrNotImplemented(err):
 		errorResponse.ErrorCode = http.StatusNotImplemented
 	default:
-		a.logger.Error("API ERROR",
-			mlog.Int("code", http.StatusInternalServerError),
-			mlog.Err(err),
-			mlog.String("api", r.URL.Path),
-		)
 		errorResponse.Error = "internal server error"
 		errorResponse.ErrorCode = http.StatusInternalServerError
 	}
 
+	a.logger.Warn("api error response",
+		mlog.Int("code", http.StatusInternalServerError),
+		mlog.Err(err),
+		mlog.String("api", r.URL.Path),
+	)
+
 	setResponseHeader(w, "Content-Type", "application/json")
 	data, err := json.Marshal(errorResponse)
 	if err != nil {