CBG-4518: better error to pass up stack when xattr config doesn't exist

[gregns1]

CBG-4518

• Pass up stack more descriptive error when metadata doc is found with no xattr config
• Added test to test different scenarios returning the new xattr error type added
• Added code to add db's ties to a bucket into invalid db tracking map when we hit this issue on config poll for a bucket.
• Added code to fail config update and take tb offline, avoiding a config rollback when this is hit during a db config update.

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

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2966/

[torcolvin]

I feel pretty nervous about changing this error still, given how the retry code works here

sync_gateway/rest/config_manager.go

Line 375 in aaa9f01

if legacyErr != nil && !base.IsDocNotFoundError(legacyErr) {

I'd make sure that we have tests in the rest package that cover each of the places IsDocNotFoundError where the error would be hit in config_manager for xattr config.

I think we should pursue writing a document describing the ways that a user can end up this scenario.