API: expose more details for writing points error of type "engine: field type conflict"

Using the client's async `WriteAPI` and `Errors()` channel, when above error happens it is impossible to relate this to the originating write since the database server does not provide more information.

It would be nice if the database server error type could expose additional information about which field experienced the error (name) and which types are in conflict. The full error information should be sent to the client.