Wrappers for DAP validator functions that throw exceptions

[bgrant]

The functions in the Distributed Array Protocol validator return a (success, errmsg) tuple; however, when we use them in DistArray, we usually want to throw an exception when a structure doesn't validate. This PR adds three functions to testing.py:

validate_dim_dict
validate_dim_data
validate_distbuffer

which wrap the corresponding DAP validator functions but throw exceptions rather than return tuples. They all use the raise_exception decorator that I added to testing.py.

[kwmsmith]

@bgrant could you add an AssertRaises test that exercises the decorator's logic?

[bgrant]

@kwmsmith : Added some simple tests.

[kwmsmith]

LGTM.