Bug 1058853 - Don't break the gallery if we have corrupt images

At the point where you're inserting this code, we haven't tried decoding the full-size image yet. We've just found that the EXIF preview is corrupt. And the image seems to work in desktop Firefox, so it doesn't appear corrupt. Why shouldn't we call useFullsizeImage() here?  Previously the thrown error was probably hanging the metadata parsing process, but now that you catch it in crop_resize_rotate.js, can't you just leave this previewerror() function alone?

Also, note that previewerror() is called in two different places, and the modification you're making here only makes sense in one of those places. I'd suggest that if these changes are needed, you make them in gotThumbnailBlob() and modify that function so that it does not call previewerror(). If you do that, then you won't even need to check the error string, which would be a nice bonus.