Propagate Connection Closed Information up to top-level (fix #465) (#545)

This PR implements a mechanism to propagate connection loss information
from the Lambda runtime client to the runtime loop, enabling termination
without backtrace when the connection to the Lambda control plane (or a
Mock Server) is lost.

The changes are:

- When the connection is lost,
`ChannelHandlerDelegate.channelInnactive()` now correctly calls
`resume(throwing:)` on the ending continuation, for all states
(`.waitingForNextInvocation ` and `.sentResponse`). This eliminates the
hangs on connection lost..

- I added top-level error handling on `LambdaRuntime._run()` 

- Add a unit test to check that either
`LambdaruntimeError.connectionToControlPlaneLost`, a `ChannelError`, or
an `IOError` is thrown when the server closes the connection

Sources/MockServer/MockHTTPServer.swift

@@ -216,7 +216,7 @@ struct HttpServer {
         } else if requestHead.uri.hasSuffix("/response") {
             responseStatus = .accepted
         } else if requestHead.uri.hasSuffix("/error") {
-            responseStatus = .ok
+            responseStatus = .accepted
         } else {
             responseStatus = .notFound
         }