No error assertions in NoMAD unit tests

[samuong]

Prior to #41, some of the unit tests for NoMAD integration contained assertions on the message within an error. With these removed, the tests just check for the presence of an error, without checking what type.

This should be changed to check the error type, using the new errors.As function, introduced in Go 1.13.

Additional context can be found in the comments of #41.

[camh-]

I would only do this if the error result is part of a stable API. It is sufficient to check that the function under test returns an error under error conditions. If the function is not documented to return a particular type of error, testing for that just makes the tests more fragile.

[samuong]

Ok that's a fair point, this is definitely not a documented API. Julia didn't seem to have a super strong opinion on this either, so I'll close this issue and we can leave the error type assertions out.