Client side parsing and i18n of TrustedApplication request errors. Add error codes to server error messages.

At present the only way for clients to determine the underlying cause of an error in a trusted application request is via the "X-Seraph-Trusted-App-Error" header.

This is a brittle mechanism as any changes to the error message format could break error handling mechanisms of receiving clients

We should implement a more solid way of reporting particular requests failure modes so that clients are insulated from String formatting changes

This needs to remain compatible compatible with older clients, and the full value should be human-readable.