fix: tests for errors fixed to prevent false positives

[marksmccann]

• The expect statement was moved out of the catch block so that it will also be hit when it succeeds.
• 'throws an error if a source file is not found' test was changed to 'succeeds silently if a valid source file is not found' because that is what's really happening; we were getting a false positive on this test before.
• Removed 'throws an error 'sourceMap' is not a valid file path' test because it is no longer relevant (we were also getting a false positive on this test before), because sourceMap now supports dirs.

closes #23

[marksmccann]

@phillipluther Just FYI, had to remove the finally block in render. I had to add a throw statement in the catch block so that it would throw errors when not using the callback. And when I added the throw statement, I got an unhandled promise rejection when using the callback syntax. I think it works great now though.