API Cleanup: renamed '_badcontinue'->'badcontinue', one die()

* If a query module has 'continue' parameter, it will auto-report
that it has 'badcontinue' error.
* Added APIQueryBase::DieUsageContinueIf( $condition ) that shows
correct badcontinue error if $condition is true.

Change-Id: I9c48bda6de0cde3c117ad24460bddf6980279633
Merge "API Cleanup: renamed '_badcontinue'->'badcontinue', one die()"