AsyncHttpClient doesn't handle errors

[yglukhov]

Which causes all sorts of weird behaviors. E.g. the following sample demonstrates an infinite hang in case when server closes connection before sending all the data. Otherwise if client is the only async "thread" in the process, the client will occasionally fail with error "No handles or timers registered in dispatcher." with totally irrelevant stack trace.

import asynchttpserver, httpclient, asyncdispatch

proc startServer() {.async.} =
  proc cb(req: Request) {.async.} =
    let headers = { "Content-length": "15"} # Provide invalid content-length
    await req.respond(Http200, "Hello World", headers.newHttpHeaders())

  var server = newAsyncHttpServer()
  await server.serve(Port(5555), cb)

proc runClient() {.async.} =
  let c = newAsyncHttpClient(headers = {"Connection": "close"}.newHttpHeaders)
  let r = await c.getContent("http://127.0.0.1:5555")
  echo r

asyncCheck startServer()
waitFor runClient()