Use stub_const() rather than setting ENV in tests

Setting ENV has side effects which we need to take care
with (hence the after(:each) step). Using stub_const stubs out
the ENV hash to allow us to be self contained within the
assertation.
Raise error if FOG_CREDENTIAL is unset
Raise error on invalid FOG_CREDENTIAL
Correct Exception classes for FOG_CREDENTIAL errors
Merge pull request #1 from alphagov/raise_error_on_missing_fog_cred

Improve bad input handling