If fail to open plugin library also say which plugin was being loaded

Added to the exception context which plugin was requested to load at the time the plugin library opening failed.
Merge pull request #6803 from Dr15Jones/sayWhichPluginLoadingWhenLibraryNotFound

If fail to open plugin library also say which plugin was being loaded
Merge remote branch 'origin/CMSSW_7_4_X' into CMSSW_7_0_X