log failure to start service (e.g., port already in use)

[zackse]

Previously, failure to bind would result in a silent failure after the "Listening on " message. This change wraps the call to ListenAndServe to report any failure.

# ./aws-signing-proxy -target https://my-endpoint.amazonaws.com -port 9999 &
[3] 26203
Listening on :9999

# ./aws-signing-proxy -target https://my-endpoint.amazonaws.com -port 9999
Listening on :9999
2019/04/19 11:51:43 listen tcp :9999: bind: address already in use