Minor action items window fixes

[mbien]

• fixed the hardcoded javac warnings filter (error key changed)
• updated the "Errors" scanner description to be more correct
• fixed the TODO scanner which didn't work if the "lineComment" field was empty (default for html, xml etc)

it isn't obvious (e.g https://github.com/apache/netbeans/issues/4790, ) that the "action items" (ctrl+6) window can't show java hints/inspections but only javac errors/warnings. This clarifies the labels a bit.