Catch the error from a client disconnect due to stalled response

[xbhatnag]

Motivation

PR #123 was hanging on presubmit for AHC conformance tests on Linux.

This was root-caused to an issue with testCancelPreHeaders and testCancelPreBody.

Here is the sequence of events:

1. testCancelPreBody cancels the task making the request to /stall_body.
2. The task cancellation causes AHC to close the connection to the server.
3. The server then throws an error from the indefinite try await Task.sleep(..) due to client disconnection.
4. This error propagates up from the server handler into NIOHTTPServer.
5. Under Linux, the server is unable to handle this correctly and subsequent tests end up failing.

Note: This issue does not happen when the tests run under Mac + AHC. Further investigation into possible server-side issues is warranted here. Why does it only occur for Linux and not Mac?

Modifications

Instead of letting the error propagate up into the server, the handler swallows any errors from the /stall and /stall_body endpoints.

Result

The AHC conformance tests in #123 now pass under Linux.

Test Plan

N/A. This fixes a broken test.

[xbhatnag]

@gjcairo can you help investigate why the server behaves differently on Mac vs Linux?