Resolves #258 (Small improvements to the error handling for the config)

[Zedjones]

I added a validator tag called "notempty" which simply checks if the string set is an empty string. This is handy for required values that are strings, since we normally don't want them to be empty. I double-checked that the validator library didn't have this functionality already, but it doesn't seem to.

So now, you get nicer messages when you have a port number that is too small or too large or when you have an empty string in a required place.

Ex.

$ go run cmd/teleirc.go
Field Settings.IRC.Port failed to validate: 65535 too large.
Field Settings.IRC.Channel was an empty string. Perhaps you had a # and need to surround the value with ""?

[jwflory]

This is handy and should be a more definite way of avoiding the issue @Tjzabel ran into last week. Thanks for jumping on this @Zedjones! Merging. 🎬