Add user defined errors for easier testing

[naterush]

Issue

Currently, if we want to test that some condition is false (e.g. that some block is not the forkchoice), we can construct a string that asserts it is true, and then test that running the testing language with this string fails an assertion. However, we cannot be sure which assert is really causing the error to fail.

See discussion #19 for more details.

Implementation

Add necessary user-defined errors (like ForkChoiceError) that allow us to check the correct part of the test is failing.

[naterush]

@djrtwo I'm not totally sold on the above. I think a better solution to this might be adding a check_not_forkchoice (or the equivalent), so we can make any checks we need and be able to continue with the test (which is not trivial when an error is thrown).

Essentially - we say that the testing language itself should never fail, but we give it the ability to check any situations it might be in. Let me know your thoughts. Might be totally off base here :)

[djrtwo]

Yeah, you're right. I hadn't thought it through all the way.

At this point, I just want to ensure TestLangCBC is correct by adding some tests. See #35