HTTPBin ignores all channel errors on the server side

[weissi]

Right now, the HTTPBin ignores all channel errors on the server side.

That's bad for multiple reasons:

• most servers that encounter protocol errors from the client just close the connection
• we can't make sure the server is happy

I think the default behaviour of HTTPBin should be that it runs an XCTFail("server error") and closes the connection. However for tests that expect server failures, this should be configurable.

Fixing this issue means:

• having HTTPBin have a configuration property(?) that can change the behaviour
• making HTTPBinHandler implement func errorCaught.