Bugfix/issue 863 bessel first kind nan

#### Summary:
Fixes issue #863.

#### Intended Effect:
Throws std::domain_error when second argument of bessel first kind is NaN. 

#### How to Verify:
Run test/unit/math/functions/bessel_first_kind

#### Side Effects:
None.

#### Documentation:
None. Should document behavior at NaN with other functions later.

#### Reviewer Suggestions:
Anyone.