Improve the error handling.

[kitsunde]

When coinbase gives a 502 and presumably any error, it throws a TypeError because it's tries to decode the returned html as json.

This whole library basically assumes coinbase is always returning the right thing, there's no error handling here at all: https://github.com/coinbase/coinbase-python/blob/master/coinbase/wallet/model.py#L301-L315

At a minimum it should return the actual connection issue, these are not type issues.

[sds]

Hey @kitsunde, thanks for the suggestion.

Would happily accept a PR implementing improved error handling!