Feature/issue 891 check symmetric nan

#### Summary:
Fix check_symmetric so it throws if any elements not on the main diagonal are nan.

#### Intended Effect:
Make sure check_symmetric fails if elements not on the main diagonal are nan. This is better because functions like quad_form_sym only call check_square, check_multiplicable, and check_symmetric. Most of the other functions usually also call check_pos_definite or something similar.

#### How to Verify:
Run test/unit/math/error_handling/matrix/check_symmetric

#### Side Effects:
None.

#### Documentation:
Added doc in the code.

#### Reviewer Suggestions:
Anyone.