Fixed incorrect forward declaration of Looper

The clang compiler (correctly) complained that Looper was forward
declared as a struct but is actually declared as a class.
Fixed typo in setError that caused error_ not to be set

clang caught a typo where the argument 'error' was being assigned
to itself rather than to 'error_'.
Merge pull request #4965 from Dr15Jones/fixClangProblemsInDataFormatsPatCandidates

Fix clang problems in DataFormats/PatCandidates
Merge remote branch 'origin/CMSSW_7_2_X' into CMSSW_7_0_X