Enhance docs with possible values of Compute operation's .error.errors[].code

[mziccard]

We should figure out possible values that can be assumed by error.errors[].code.
The field is only documented as:

The error type identifier for this error.

[garrettjonesgoogle]

This has been added to our feature backlog: https://github.com/GoogleCloudPlatform/google-cloud-java/wiki/Feature-backlog . This issue will be closed but is linked in the backlog and can continue to be used for comment and discussion.