error handling consistency

[dwlehman]

Establish some conventions for error handling, document them, and implement them.

Some ideas:

• don't raise built-in python exceptions like ValueError
  • expand blivet.errors as needed
• when to raise an exception versus returning an error code
  • return values would make unit testing cleaner/easier than exn regex
• when/whether to log exceptions (before raise -v- in except)
• audit exception messages for clarity/accuracy
  • blivet.partitioning could use some improved clarity and accuracy

[vpodzime]

we need to come up with some nice way how to deal with errors coming from libblockdev too -- they are all GLib.GError with no easy way how to catch them granularly