lisa.trace: Raise MissingTraceEventError from Trace.df_events()
lisa.trace: Raise MissingTraceEventError from Trace.df_events()

MissingTraceEventError provides better reporting when it comes to
events.

Make MissingTraceEventError inherit from ValueError to avoid a
compatibility break.

ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa