Report data file errors in more detail

[bartfeenstra]

This PR fixes https://github.com/nedbat/coveragepy/issues/1781

Changes

• Introduce NoDataError subclasses for specific purposes
  • Factory methods to construct helpful messages, because it keeps all the default behavior around assumptions of exceptions just having a single string argument working (it gets funky when pickling and what not, so I tend to take this approach)
• When possible, suggest that the user runs the combine command
• All critical (albeit sometimes seemingly trivial) bits are now centralized, e.g. no more duplication of messages. This makes it easier to keep everything in sync and improve on.

[nedbat]

Thanks for doing this. I would like to include the "coverage combine" message only if there are combinable data files. I've included other comments in the diff.