GODRIVER-1802 add ServerError interface

[iwysiu]

Still not sure about add Unwrap to the interface when CommandError is the only one that actually uses it.

[kevinAlbs]

Looking good! Just have a few questions. I'm not completely sure about the usefulness of Unwrap either.

[kevinAlbs]

LGTM!

[divjotarora]

Code changes look good! I'm really happy with how this new API is coming together. The main thing missing here is documentation. There is a planned effort for the docs team to create reference docs for the driver, but we don't know when that'll be, so we should at least add a section to mongo/doc.go. I'm fine making this a follow-on ticket, but it should at least be a requirement for this project.

[iwysiu]

added a little reference to ServerErrror to mongo/doc.go. I can make it a more full fledged section if that seems useful, but I thought that was enough to get users to look if they need it.

[divjotarora]

I think the docs for the ServerError type in mongo/errors.go should mention that it can represent one or more errors (e.g. for use-cases like BulkWrite or InsertMany, it can represent a number of write errors and/or a write concern failure) and that some of the interface functions like HasMessage will return true if any of the individual errors satisfies the check.

[benjirewis]

LGTM!! 🎊