datastore: Client.Close returns error.
datastore: Client.Close returns error.

BREAKING CHANGE. DO NOT SUBMIT.

Close methods should always return error. And there is an actual error
to return here, the one from the underlying connection Close.

Fixes #373.

Change-Id: Ia37c8b11b1d104e46b6114da53e1010a0573bad7
Reviewed-on: https://code-review.googlesource.com/8891
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go