The histogram validator and other probe parsers printed a single error at a time and halted the build.
Extended the ParserError() class to support printing multiple errors at a time.
Changed the parse_histograms.py histograms validator and partitioned all ParserErrors into 'immediately fatal'
and 'eventually fatal'. Eventual errors are now accumulated and printed at one go upon exit.
Future work might involve changing the other probe parsers (scalars and events) to make use of the extended functionality.
