Improve error message for config parsing error

[nevo]

It's not a big deal for developers, but for ordinary user, when you change some config name to unexpected/unknown string, the error message is not easily understandable.

for example, you change "http-addr" to "this is bad name" in config.yaml, then you'll get the error:

Nevos-MacBook-Pro:gosnow nevo$ uniqush-conn -config=config.yaml -key=key.pem -port=8964 Config error: [service=this is bad] Service information should be a map

we probably should say something like:

Config error: unknown service name: this is bad

[nevo]

Oops, I clicked the wrong button - I reopened it...

[monnand]

More thought: We should add the line number as well. Let me see if I can fix this. Since we are using a third party library to parse the config file, I'm not sure if we have the flexibility to do such thing.