Add documentation about reporting error codes from within PostgreSQL

Here's what I use on some projects (most error codes have been omitted because they won't make sense for your app):

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Error codes
===========

Errors from PostGraphile may come through with one of these error codes.

Error code rules
----------------

Some error codes are [reserved by
PostgreSQL](https://www.postgresql.org/docs/9.6/static/errcodes-appendix.html),
so error codes we use must match these criterea:

- 5 alphanumeric (capitals) letters
- First character must be a letter
- First character cannot be F, H, P or X.
- Third character cannot be 0 or P.
- Fourth character must be a letter.
- Must not end `000`

Rewritten, the above rules state:

- Char 1: A-Z except F, H, P, X
- Char 2: A-Z0-9
- Char 3: A-Z0-9 except 0, P
- Char 4: A-Z
- Char 5: A-Z0-9

Common
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- SELF0: you can't do that to yourself (e.g. remove admin from yourself)
- ERROR: invalid arguments
- UNSUB: error unsubscribing - perhaps your token was wrong?