[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
[ticket/12381] Fix broken error message when selecting invalid DB driver

PHPBB3-12381
Merge pull request #2279 from Elsensee/ticket/12381

[ticket/12381] Fix broken error message when selecting invalid DB driver

* Elsensee/ticket/12381:
  [ticket/12381] Fix broken error message when selecting invalid DB driver
Merge remote-tracking branch 'marc1706/ticket/10687' into develop-olympus

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