Rethink exception handling

[careo]

Attributor has a few huge pain points in its exception handling. The biggest offender probably being Attribute#load, who loves to eat errors that come from lazy attribute creation and have nothing to do with the actual loading at hand.

I can think of at least two fixes (both should be done):

1. add a separate hierarchy of exceptions that specifically does not derive from StandardError for those exceptions that are fatal and a result of a misconfiguration.
2. rework the myriad Type.load to not rely on exceptions like this one for reporting malformed data: ArgumentError: invalid value for Integer(): "hi"

[careo]

This is being reworked as part of praxis/praxis#261.