CBG-3921 use IsDocNotFoundError everywhere

[torcolvin]

• switch IsKeyNotFoundError and IsError(sgbucket.KeyNotFoundError)
• slightly modify the behavior of IsDocNotFoundError to account for errors that might be wrapped without being the lowest level error.

couchbase/sg-bucket#126
couchbaselabs/rosmar#35

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2430