surface panic when calling Finish is implicit

[codyoss]

When Finsh is called from Cleanup, it is not done so in a deffered
context. Because of this, we can't recover from panics. The
testing package will re-surface any panics but only after the
Cleanup functions is executed. Because we were calling t.Fatal we
were hiding panics from users of the library when they were using
Finish implicitly. Fatal calls runtime.Goexit() under the hood so
the panic never gets the chance to be handled by the testing
package.

Also, removed logging as after thinking about it more it seems a
little aggressive to push the feature onto users.

Note, when there is a panic and Finish is called from the Cleanup
function that a few extra lines will be logged from the calls to
ErrorF. This seems like a small price to pay to get the context
of the panic

Fixes: #428