Add tests for expected errors

[bwspenc]

Reason

We need to make sure we have good coverage in our testing of error conditions. Add tests of error conditions. Also, some of our existing code has calls to mooseError() for conditions that are more appropriate for mooseAssert() because they are internal code consistency checks, and cannot be reached by a user.

Design

Switch some instances of calls to mooseError() to mooseAssert() and add error tests.

Impact

No impact on existing usage, just more testing.