[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
[ticket/12397] Fix sql_unique_index_exists doc block

db_tools::sql_unique_index_exists() searches also for unique indexes but not
primary key indexes.

PHPBB3-12397
Merge pull request #2310 from n-aleha/ticket/12397

[ticket/12397] Fix sql_unique_index_exists doc block

* n-aleha/ticket/12397:
  [ticket/12397] Fix sql_unique_index_exists doc block