Add LocalProtocolError, RemoteProtocolError

[tomchristie]

Distinguish between local and remote protocol errors, so that we can expose local errors for invalid requests, eg...

>>> import httpcore
>>> http = httpcore.SyncConnectionPool()
>>> http.request(b"GET", (b'https', b'example.org', 443, b'/'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 153, in request
    method, url, headers=headers, stream=stream, timeout=timeout
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection.py", line 93, in request
    return self.connection.request(method, url, headers, stream, timeout)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/http11.py", line 61, in request
    self._send_request(method, url, headers, timeout)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/http11.py", line 88, in _send_request
    event = h11.Request(method=method, target=target, headers=headers)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/contextlib.py", line 130, in __exit__
    self.gen.throw(type, value, traceback)
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_exceptions.py", line 12, in map_exceptions
    raise to_exc(exc) from None
httpcore._exceptions.LocalProtocolError: Missing mandatory Host: header

And...

>>> import httpcore
>>> http = httpcore.SyncConnectionPool()
>>> http.request(b"GET", (b'https', b'', 443, b'/'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_sync/connection_pool.py", line 130, in request
    raise LocalProtocolError("Missing hostname in URL.")
httpcore._exceptions.LocalProtocolError: Missing hostname in URL.

[florimondmanca]

Sweet!

I have a question - do we even need ProtocolError anymore? I don't think it's being used elsewhere in the package anymore. Edit: okay, ProtocolError is kept as the base class, that makes sense and I hadn't seen that. :-)