Allow specification of "@fatal_exceptions"

When a fatal exception is raised the job will be immediately marked as
"failed" (this is the inverse of "@retry_exceptions").

High level changes:

* Ensure that only one of the two exception handling stategies is
  specified (e.g. "@fatal_exceptions" or "@retry_exceptions")
* Favor checking "retry_exceptions" over "fatal_exceptions" as it has
  been part of the API for a longer time
* Add tests to cover the validation and feature behavior
* Update documentation
Merge pull request #94 from jzaleski/jwz-fail-fast-for-specific-exceptions

Allow specification of "@fatal_exceptions"