Do not write on a closed transport

[yohanboniface]

Open questions:

• should we try to write and catch RuntimeError instead?
• should we log? (but we don't have any logger yet nor logger configuration, see next point)
• should we call on_error (but write is not yet async so it cannot call hooks yet)

See also:

• uvloop: after CancelledError web response still tries to write EOF to closed socket aio-libs/aiohttp#1790
• RuntimeError: unable to perform operation on UnixTransport sanic-org/sanic#1130

Traceback:

Jun 10 03:12:50 scw-8f883c Trefle[6872]: Exception in callback <bound method HTTPProtocol.write of <roll.HTTPProtocol object at 0x7fce04d2f6c8>>
Jun 10 03:12:50 scw-8f883c Trefle[6872]: handle: <Handle HTTPProtocol.write>
Jun 10 03:12:50 scw-8f883c Trefle[6872]: Traceback (most recent call last):
Jun 10 03:12:50 scw-8f883c Trefle[6872]:   File "uvloop/cbhandles.pyx", line 64, in uvloop.loop.Handle._run
Jun 10 03:12:50 scw-8f883c Trefle[6872]:   File "/srv/trefle/venv/lib/python3.6/site-packages/roll/__init__.py", line 522, in write
Jun 10 03:12:50 scw-8f883c Trefle[6872]:     self.transport.write(payload)
Jun 10 03:12:50 scw-8f883c Trefle[6872]:   File "uvloop/handles/stream.pyx", line 636, in uvloop.loop.UVStream.write
Jun 10 03:12:50 scw-8f883c Trefle[6872]:   File "uvloop/handles/handle.pyx", line 165, in uvloop.loop.UVHandle._ensure_alive
Jun 10 03:12:50 scw-8f883c Trefle[6872]: RuntimeError: unable to perform operation on <UnixTransport closed=True reading=False 0x7fce04d09048>; the handler is closed