Document exceptions

まだ誰も着手していません。

Please add the asyncpg exceptions to the documentation. I have no trouble reading source code, so was pleasantly surprised to find how rich the included exception classes are.

```python
>>> exceptions = sorted((o for o in vars(asyncpg).values() if isinstance(o, type) and issubclass(o, Exception)), key=lambda o: o.__name__)
>>> for exc in exceptions:
... if getattr(exc, 'sqlstate', None): continue # looong list otherwise
... print(exc.__name__)
... if exc.__doc__: print(exc.__doc__)
... print()
...
FatalPostgresError
A fatal error that should result in server disconnection.

InterfaceError
An error caused by improper use of asyncpg API.

InterfaceWarning
A warning caused by an improper use of asyncpg API.

InternalClientError
All unexpected errors not classified otherwise.

OutdatedSchemaCacheError
A value decoding error caused by a schema change before row fetching.

PostgresError
Base class for all Postgres errors.

ProtocolError
Unexpected condition in the handling of PostgreSQL protocol input.

UnknownPostgresError
An error with an unknown SQLSTATE code.

>>> # generated exceptions for all possible sqlstate values
...
>>> sum(1 for exc in exceptions if getattr(exc, 'sqlstate', None))
240
```

asyncpg の例外クラスと issue に示されている例外一覧から始め、基底例外および API 例外と SQLSTATE によって生成される例外を分けます。利用可能なクラスとその説明を、SQLSTATE のカバレッジを含めて文書化します。プロジェクトのドキュメントで例外リファレンスを網羅できれば完了です。

索引モデルが issue の本文から書いたものです。