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

[wahabmk]

Signed-off-by: Wahab Ali [email protected]

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

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

References
N/A