Bugfix/issue 1179 exponential log bounds checking

#### Summary:

Modified exponential log density function to throw an exception if evaluated at a negative y value. 

#### Intended Effect:

1. exponential_log throws a domain error when y < 0

2. src/test/unit-distribution/continuous/exponential/exponential_test.hpp invalid_values tests invalid y values

#### How to Verify:

src/test/unit-distribution/continuous/exponential/exponential_test.hpp

#### Side Effects:

None.

#### Documentation:

None. Doc already says the function should throw std::domain_error at negative values. 

#### Reviewer Suggestions:

Any