Document exceptions

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.

>>> 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

从 asyncpg 的异常类和 issue 中列出的异常列表开始，将基础异常和 API 异常与由 SQLSTATE 生成的异常分开。记录可用的类及其描述，包括 SQLSTATE 的覆盖范围；当项目文档涵盖异常参考时，即视为完成。