Improved some thumb.php error messages.

Change-Id: I5a61e898dd97db67b719a13ca044530c4baecc99
[FileBackend] Fixed bug where "false" could be included in getDirectoryList().

Change-Id: Ib14192d4f5c5143b07af2191b62ea3919483445a
Merge "Improved some thumb.php error messages."
Merge "[FileBackend] Fixed bug where "false" could be included in getDirectoryList()."