Remove imprecise catch-all exception handler
implement bug() to distinguish software errors from run-time errors (issue #34)

This should hopefully encourage users to take the appropriate action.

https://github.com/aspiers/ly2video/issues/34
getAbsolutePitches(): fix bug with bug-reporting
getAbsolutePitch(): tidy up error message
output divider line before fatal errors
getTopAndBottomMarginSizes(): tidy up error message