Add missing gateway exceptions

[charlie-s]

I'd like to issue a PR for a patch that fills out additional exception mappings. Specifically, there are gateway exceptions that are missing.

Before I can do so, I was wondering what defines mapping_type, mapping_code, and mapping_message?

Use case: I'd like to have better error handling on my end. For example, when a transaction is requested and the transaction cannot be found, no mapping is found and I get a very generic error return new Error(gatewayRspMsg);.