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

Comienza con las clases de excepciones de asyncpg y la lista de excepciones mostrada en el issue, separando las excepciones base y de API de las excepciones generadas por SQLSTATE. Documenta las clases disponibles y sus descripciones, incluida la cobertura de SQLSTATE; se considera terminado cuando la referencia de excepciones está cubierta en la documentación del proyecto.