Replaced try...catch with ParameterSet::existsAs calls

This code was causing dozens of exceptions to be thrown and then
caught in production jobs. Since the code was only testing the
existence of a ParameterSet it was easy to replace with existsAs.

NOTE: It looks to me like this code is completely unnecessary.
The loop just before this one goes over the exact same parameter names
and asks for them as tracked parameters. If that succeeds, they can't
also be untracked so the code modified does nothing. If that fails
an exception is thrown and the code modified is never called.
Merge pull request #2772 from Dr15Jones/avoidTryCatchInHLTHiggsSubAnalysis

HLTriggerOffline/Higgs -- Replaced try...catch with ParameterSet::existsAs calls