Give better error messages in image-convert

* lisp/image/image-converter.el (image-convert): Make
image-convert bug out earlier on a wrong IMAGE-FORMAT value
(bug#38310).

diff --git a/lisp/image/image-converter.el b/lisp/image/image-converter.el
index b4d10c861b06156eba644cb51f9d787ac6dd7421..1230b6b9cbf36a8c05eb75e55745da0c6c95c70a 100644 (file)
--- a/lisp/image/image-converter.el
+++ b/lisp/image/image-converter.el
@@ -87,6 +87,9 @@ data is returned as a string."
     (image-converter--find-converter))
   (unless image-converter
     (error "No external image converters available"))
+  (when (and image-format
+             (not (= (length (split-string (symbol-name image-format) "/")) 2)))
+    (error "IMAGE-FORMAT should be a symbol like `image/png'"))
   (with-temp-buffer
     (set-buffer-multibyte nil)
     (when-let ((err (image-converter--convert