Wrapping gRPC "already exists" with a HappyBase exception.

[dhermes]

Fixes #1818.

NOTE: For now I just used a happybase import so that I could get the tests passing locally via nosetests. However, these tests will fail on Travis. I don't think we should take on the Happybase library as a dependency. Maybe @tswast has some opinions?

Potential option: try to import it (like we do with pytz / pandas / etc.) and then use it if it's there for the "native" exception

/cc @tswast

[tseaver]

We could fall back to gcloud.exceptions.Conflict (which maps the HTTP 409 status code used to signal the same condition in RESTy apps).

[dhermes]

@tseaver Implicit in that "fall back" is that you agree with the try import approach?

[tseaver]

Yup, Something like:

try:
    from happybase.hbase.ttypes import AlreadyExists
except ImportError:
    from gcloud.exceptions import Conflict as AlreadyExists

[tswast]

@dhermes Yes, I think requiring a happybase dependency is yucky. Since the exception type matters more for someone who is migrating an app, it seems okay to have them add that as a dependency since they are already likely to have the build environment needed for the Thrift stuff.

Falling back to gcloud.exceptions #1843 (comment) seems like a good option. I'm not very fond of exposing the grpc errors directly.

[dhermes]

@tseaver @tswast PTAL

[tswast]

LGTM. Thanks!

[tswast]

Looks like there are some lint errors in Travis.

[dhermes]

I can't get the wrong-import-order to trigger on my local install. This happened before in gcloud._helpers and I couldn't repro on OS X, GCE or my two Linux boxes. I'll move the six import and see if there is any change.

[dhermes]

OK that fooled it.

@tseaver Am I good to merge?

[tseaver]

LGTM