Make file loading more resilient

See (PICARD-605)[http://tickets.musicbrainz.org/browse/PICARD-605]
Fix evaluation of script functions with zero arguments.
Fixed plugin precedence for the first plugin in plugin list.

If a plugin was in both the system wide and ther user's plugin folder and the plugin was the first in the list the plugin was loaded and listed twice.
Load the options dialog even if there is a ScriptError.

Before a ScriptError, which is rasied for e.g. a missing function, the options dialog would not show up. This prevents the user from even fixing the scripting.
Merge pull request #346 from phw/plugin-fixes

Fixed plugin precedence for the first plugin in plugin list.
Merge pull request #347 from phw/fix-load-options-on-script-error

Load the options dialog even if there is a ScriptError.
Update picard.pot file
Update translation files
Better fix for script functions with zero arguments.
Merge pull request #345 from phw/fix-script-function-with-zero-arguments

Fix evaluation of script functions with zero arguments.
Reset self.form_values after generating the HTML file

Otherwise, :qa
Fix the last commit

because I forgot to `git add` the file again
PICARD-418: fallback to using release group cover

It adds the option to use release group cover art from CAA.
It is required to enable CAA and release group fallback is disabled by default
Images can have other types (png, jpg, gif), remove '.jpg'
Fallback -> Fall back
Move fallback option to cover archive options group
Honour CAA image size preferences (full/250/500)
Remove useless line after acab1dc
Be sure to continue on next provider if no front image is found.
Make CoverArtProviderCaaReleaseGroup inherits from CoverArtProviderCaa

It solved many issues, re-use json parsing code, and handling of various caa options (types, sizes, etc..)