added counter member to 'DataMissingEvent' exception

- allows more fine-grained handling of error (e.g. retry if missing events below a certain thresh.)
- access via DataMissingEvent::numberMissing
- addresses issue psi46/pxar#92
fills number of missing events into 'DataMissingEvent' exceptions

- addresses issue psi46/pxar#92
make use of DataMissingEvent::numberMissing information

- shows number of missing events in dialog box if exception occurs
- fixes psi46/pxar#92