Expanded exceptions gcloud and its storage sub-module.

Not sure what direction to expand exceptions in, should I map them directly to errors referenced form here https://developers.google.com/storage/docs/reference-status ?

Or group 
InvalidBucketNameError
BucketNameUnavailableError
KeyTooLongError 
and simalar

Into something like StorageCreateError/S3CreateError in boto.

Todo could use a little fleshing out, but any guidance please?
Minor nit but do we want to say `GCloud` or `Gcloud`? @marcgel WDYT? (since presentation of words/language is your thing)