Discovery - Document other HTTP response codes for error scenarios

It is possible that a TEA Client may have cached a TEA Server for a TEI, or in talking with a TEA Service, there are errors encountered.

It is suggested that other HTTP response codes are documented and their purposes within TEA defined.

For discussion - above and beyond what is already defined in the Beta 1 API Specification and the standard HTTP specification:

| HTTP Response Code | Permitted ? | Description |
| ---------------------- | ------------ | ----------- |
| 400 | Y | |
| 401 | Y | Not authorized |
| 402 | ? | Payment Required - might be implemented by Commercial TEA Services |
| 403 | Y | Authenticated, but not authorized |
| 404 | Y | To be defined - see also #163 |
| 503 | Y | Can be used to tell a TEA Client that the TEA Service no longer supports the requested TEI |

Others are welcome to add / discuss please.