Revise images controller for better compatibility with current Dragonfly.

The 'create' action of 'Refinery::Admin::ImagesController' was trying to rescue
'Dragonfly::FunctionManager::UnableToHandle', which has been removed from
the current version of Dragonfly.

This patch instead rescues 'NotImplementedError', which is what 'Dragonfly::FunctionManager::UnableToHandle' used to subclass. Whether this is the correct thing to try to rescue is just a guess, as I haven't tried to determine what errors are actually issued by the current version of Dragonfly.