LanguageResources that cannot be used: Improve handling

Current state :

• in error-log: "Unable to use :xyz as search resource."
• for pretranslation: the pretranslation returns nothing, but does not return an error, too.

Target state:

• implement a warning in the error-log
• send an email to the PM (not sure if this is done already; pls check)
• deliver the pretranslation using the other LanguageResources as if nothing was wrong (for example: if we have 10 LanguageResources assigned and 2 of them fail, then use the other 8 resources and return the result as if only those 8 would have been assigned)

In addition, pls check if this is related to the problem of "Trying to get property 'rows' of non-object" for a GET-request on editor/languageresourcetaskassoc.