Conflict error should expose the name of a field that violates a unique constraint.

[wopol]

Currently, the package exposes the driver.IsConflict function, which allows us to check whether conflict occurs, but the only way to check which field caused that error is to parse the error message. It is not good and error-prone. Is it possible to add explicitly the conflicti_keys to the ArangoError struct? Or maybe you could propose another solution that allows clients to check field names.