DRTVWR-418: Remove rogue getMessage() from llsecapi.h exceptions.

The LLProtectedDataException and LLCertException exception classes didn't used
to be derived from std::exception, so they followed their own getMessage()
convention instead of the standard what() convention. Now that they're derived
from std::exception, remove getMessage() and change its few consumers to use
what() instead. Thanks NickyD for suggesting.