Bugfix/issue 864 modified bessel first kind nan

#### Summary:
Fixes issue #864 .

#### Intended Effect:
Throws std::domain_error when second argument of modified bessel first kind is NaN. Before, if the second argument was NaN and the first argument was > 1, then Boost gave an error message like 
```
unknown file: Failure
C++ exception with description "Error in function boost::math::bessel_jy<e>(e,e) in CF1_jy: Series evaluation exceeded 1000000 iterations, giving up now." thrown in the test body.
```

#### How to Verify:
Run test/unit/math/functions/modified_bessel_first_kind

#### Side Effects:
None.

#### Documentation:
None. Should document behavior at NaN with other functions later.

#### Reviewer Suggestions:
Anyone.