Add some error logging in Listener.Serve().

[werbenhu]

When I use an incorrect WebSocket address (for example, go run main.go -ws=1882 where there is a missing colon before '1882'), it's challenging to pinpoint and discover where the problem lies. The server appears to be running fine. I suggest adding some error logging in Listener.Serve() to assist with troubleshooting.

[coveralls]

Pull Request Test Coverage Report for Build 6285127154

• 25 of 25 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 98.908%

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Totals
Change from base Build 6277994351:	0.003%
Covered Lines:	5435
Relevant Lines:	5495

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💛 - Coveralls

[dgduncan]

Should this be added to every listener? It seems like it would make sense that we bubble up the same error on every listener to make sure that the same issue does not effect the others.

[werbenhu]

@dgduncan As it stands now, I believe we should, at the very least, add error logging within the Serve() methods of the WebSocket and HTTPStats listeners.

[mochi-co]

Looks good to me, disregarding the docker build errors as that's my fault 😄