Bugfix/issue 898 check unit vector nan

#### Summary:
Fix check_unit_vector (issue #898).

#### Intended Effect:
Make sure check_unit_vector fails when NaN is passed in.

#### How to Verify:
Run test/unit/math/error_handling/matrix/check_unit_vector

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.