Bugfix/issue 543 log prob grad exception spew

#### Summary:

Fixes issue #543, which was that models spewing error messages due to log prob grad failures

#### Intended Effect:

See summary.  

Updates position of negation inside `base_hamiltonian.hpp` to be inside the try block instead of outside of it.


#### How to Verify:

Issue #543 has a model, data, and command that were broken and now work.

see:  https://github.com/stan-dev/stan/issues/543 

#### Side Effects:

None.

#### Documentation:

It's user facing, but current doc is correct.

#### Reviewer Suggestions:

Michael.