Return ErrAborted in Mutate()

Moved from GitHub dgo/46

Posted by srfrog:

The Mutate() method should return y.ErrAborted when the transaction is aborted for retry. Currently it is returning the gRPC status error which is difficult to catch by a caller. In comparison, Commit() is correctly returning y.ErrAborted when it happens.

Reference: How to catch ErrAborted in dgraph go client - #4 by dmai