add always hooks

[jarednielson]

Adds 'always' hooks which run in an ensure block in perform.

We found this feature useful for certain clean up tasks that we always want to run.
Often we would follow the pattern

def self.on_failure_zzz_cleanup(exception, *args)
  cleanup!
end

def self.after_perform_zzz_cleanup(*args)
  cleanup!
end

With an always hook this can be simplified to

def self.always_cleanup(*_args)
  cleanup!
end

There are test cases around what error messages are propagated when:
Just the code in the always hook raises an error
The code in perform and an always hook raise errors
The code in perform, on_failure hooks and always hooks raise errors