Add TransactionConflictError (#2431)
Add TransactionConflictError (#2431)

TransactionConflictError is needed to make checking for retryable
transaction errors simple.

See also the discussion following
https://github.com/edgedb/edgedb-go/pull/123#pullrequestreview-625664587
edgedb/edgedb,edgedb/edgedb,edgedb/edgedb