Fix uninformative error message when uploading unsupported image files (#13540)

Attempting to upload image files that the browser is unable to load results
in “Oops! An unexpected error occurred.”

This commit changes the error handling so that an unprocessable image results
in the file being sent anyway, which might cover a few corner cases, and
provide a slightly better error message.

app/javascript/mastodon/utils/resize_image.js

@@ -138,7 +138,7 @@ const resizeImage = (img, type = 'image/png') => new Promise((resolve, reject) =
     .catch(reject);
 });
 
-export default inputFile => new Promise((resolve, reject) => {
+export default inputFile => new Promise((resolve) => {
   if (!inputFile.type.match(/image.*/) || inputFile.type === 'image/gif') {
     resolve(inputFile);
     return;
@@ -153,5 +153,5 @@ export default inputFile => new Promise((resolve, reject) => {
     resizeImage(img, inputFile.type)
       .then(resolve)
       .catch(() => resolve(inputFile));
-  }).catch(reject);
+  }).catch(() => resolve(inputFile));
 });