Job class should throw an exception if it can't load the class

[robbiehudson]

The Job class perform method should throw an exception if it can't load the worker class. At the moment safe_str_to_class returns NoneType which then causes an error which doesn't make it immediately obvious what the problem is.

[binarymatt]

hopefully I will get to work on this sometime today

[yashh]

I think the safe_str_to_class (or) my_import should raise a error and fail with a message that pyres could n't import your Task Class. Try setting your PYTHONPATH or DJANGO settings variable.

[binarymatt]

ok, i've put a first stab up on my repository in a branch named import_error. Right now all the tests pass, but I am going to run through a smoke test tonight or tomorrow.

[binarymatt]

this has been pushed up into master