Added helper func to check if err is PK too long error

[wahabmk]

Signed-off-by: Wahab Ali wahabalimk@gmail.com

Reason for the change Ease of use for users consuming go-rethink API.

Description This PR adds a helper function that checks if error returned by RethinkDB is a Primary Key too long error. RethinkDB has a unique limitation on length of PK, so I think having this helper useful for consumers of go-rethink API, especially if they are new to RethinkDB.

Code examples N/A

Checklist

• [x] I have read and agreed to the RethinkDB Contributor License Agreement

References N/A