stats/grammar: Raise meaningful error when asked to parse bad event
stats/grammar: Raise meaningful error when asked to parse bad event

If you typo a trace event name in a call to Parser.solve, you currently get a
KeyError from trying to access the result of globals(); the stacktrace goes
through a pyparsing callback to str_to_attr via _pre_process ("""Pre-process
accessors for super-indexing""")...

Add an exception with a message to hint at what went wrong.

bjackman/trappy,ARM-software/trappy,joelagnel/trappy,ARM-software/trappy,joelagnel/trappy,bjackman/trappy,joelagnel/trappy,ARM-software/trappy,joelagnel/trappy,bjackman/trappy,bjackman/trappy,ARM-software/trappy