Add a decorator for protocol validator functions.

Wrapped functions throw an exception rather than returning a success value.
Use the exception thrower in our DAP testing.
Remove an unused import.
Return the validator tuple on success.

Because why not?
Merge branch 'master' into feature/exception-throwing-protocol-validator
Add simple tests for DAP validator wrappers.

Incidentally, add the test file `distarray/tests/test_testing.py`.
Merge pull request #329 from enthought/feature/exception-throwing-protocol-validator

Wrappers for DAP validator functions that throw exceptions