Document CBL error and status codes

Users are finding it difficult to decipher the status codes that are output by the logs
For instance "CouchbaseLite Sync ERROR:

{Push#1}

Proposed rev ‘list::demo’ #6-63a0637e496d056164811965905505462dd270fe (ancestor 2-ba6890b59df646bc3943c4c33c0f6094b9a2f2fc) rejected with status 409 ".

The status code of 409 is not defined anywhere.

The list of error codes is specified in https://github.com/couchbase/couchbase-lite-ios/blob/0380d2d2909eea0c29436d2952df97d7ef74002d/Objective-C/CBLErrors.h . But this does not directly map to the codes that are logged by system .
Hence need a convenient way for users to determine what the reported status codes mean.

(Reported by Priya Rajagopal)