Updating warning from check_consistent_size

#### Summary:

Fixes #223. Updates rejection warning to something sensible.

#### Intended Effect:

When this error pops up, the error message will be clearer.

#### How to Verify:

There is an additional unit test in src/test/unit/math/error_handling/check_consistent_size_test.cpp. 

#### Side Effects:

No. None of the logic was changed.

#### Documentation:

No documentation necessary. We haven't doc-ed our error messages.

#### Reviewer Suggestions:

Anyone. (Hopefully I've got Jenkins set up correctly by now and all the tests will run automatically.)