[ticket/10687] Improve error message if unable to get image dimensions

The error message that is presented if getimagesize() was unable to determine
an image's dimensions now additionally states that this might be caused by an
incorrect URL.

PHPBB3-10687
Merge remote-tracking branch 'marc1706/ticket/10687' into develop-olympus

* marc1706/ticket/10687:
  [ticket/10687] Improve error message if unable to get image dimensions