hpcviewer needs to improve the error messages and check saved states

Reported by @jmellorcrummey :

• The first time I ran it, it crashed because it was unhappy with the saved state in my ~/.hpctoolkit/hpcviewer directory. When I removed that, it worked OK.
  • rather than crashing, it should say that “hpcviewer failed to read saved user state from prior sessions. Your saved state may be from an older version of hpcviewer. Usually, this problem can be fixed by removing ~/.hpctoolkit/hpcviewer.”
• When I ran it on a measurement directory, it popped up with a window saying that it was a measurement directory rather than a database. However, the window looked very concerning as it said “internal error”. The viewer didn’t crash, and I was able to open a proper database directory.
  • It shouldn’t say “internal error” if it was just a user error in pointing it at a directory that isn’t a database.