[TASK] Throw specific exception if Flux template file not found

This will allow Flux and others to grab this specific Exception type by code, suppressing it or presenting a custom error message as TYPO3 flash message.
[TASK] Render fallback structure "NoTemplate" on missing file

Note: still needs to handle a case of a NULL file which can be caused by a missing selection - in which case we need to render a slightly different message.
[FEATURE] Special case fallback structure for NULL filenames

This case allows a custom message to be rendered if no Flux form was selected or otherwise configured (in other words, if a ConfigurationProvider has returned a NULL filename for whatever reason). Hope the German translation bit is intelligible…
[TASK] Leave the German to the Germans
[TASK] Add pre tag to variable dump
[DOC] Set proper return values
Merge pull request #113 from dreipunktnull/master

[TASK] Increase legibility of error message
[TASK] Add translated label
[TASK] Translate sheet label
Merge pull request #114 from dreipunktnull/master

[TASK] Make label of 'Options' sheet translatable
Merge pull request #111 from NamelessCoder/errortolerance

[TASK] Throw specific exception if Flux template file not found