refactor exceptions

Exceptions should be cleaned up before %v0.1 as these are backward incompatible changes:

  • exit_codes should be normalized across available 127 values in a logical way for easy exit code
  • auto-generated docs about possible errors should be added to make it easy to translate between apkg CLI exit code and corresponding exception/error message
  • remove unused exceptions
  • rename exception module to ex to avoid long lines
Edited by Jakub Ružička