String representation of ServerDisconnectedError should be more meaningful

[JenSte]

Long story short

When the server closes its side of the socket, a ServerDisconnectedError is raised in the client (happened to me while testing an application). Converting this exception to a string results in an empty string (or in None in earlier versions, IIRC), which is not very useful, for example when logging the exception.

Expected behaviour

The string representation of ServerDisconnectedError should always be meaningful. If no further information is available, something like "Server disconnected" could be returned.

Actual behaviour

Converting the exception to a string results in an empty string.

Steps to reproduce

#!/usr/bin/env python3

import aiohttp
import asyncio
import io
import platform
import sys


async def server():
    # Server that shuts down its write side of the connection immediately.

    async def handler(reader, writer):
        print('closing write side')
        writer.write_eof()
        await asyncio.sleep(1)

    server = await asyncio.start_server(handler, '127.0.0.1', 8080, limit=20)
    await server.serve_forever()


async def main():
    asyncio.create_task(server())
    await asyncio.sleep(1)

    data = b'0' * 30
    file = io.BytesIO(data)

    try:
        async with aiohttp.ClientSession() as session:
            print('making request')
            await session.patch('http://127.0.0.1:8080', data=file)
    except Exception as e:
        print(f'type(e): "{type(e)}"')
        print(f'str(e): "{str(e)}"')


if __name__ == '__main__':
    print(f'platform: {platform.platform()}')
    print(f'python version: {sys.version.replace(chr(10), "")}')
    print(f'aiohttp version: {aiohttp.__version__}\n')

    asyncio.run(main())

$ ./test.py 
platform: Linux-5.2.18-200.fc30.x86_64-x86_64-with-fedora-30-Thirty
python version: 3.7.4 (default, Jul  9 2019, 16:32:37) [GCC 9.1.1 20190503 (Red Hat 9.1.1-1)]
aiohttp version: 3.6.2

making request
closing write side
type(e): "<class 'aiohttp.client_exceptions.ServerDisconnectedError'>"
str(e): ""

Your environment

Please see the output of the script above.

[asvetlov]

The fix is welcome!