Add a more helpful error message when unkown options are encountered

[caldwell]

When there is a bad option, docopt-go silently prints the usage and gives no indication of which option(s) it didn't like. It's very confusing when a program with a lot of options has a command line with one of them misspelled.

This patch complains about unknown options instead of silently printing the usage.

[coveralls]

Coverage increased (+0.08%) to 88.344% when pulling 871d908 on caldwell:unknown-option-err into ee0de3b on docopt:master.

[artburkart]

I just ran into this, so now my go.mod looks like this:

module github.com/username/project-name

require (
    github.com/caldwell/docopt.go v0.0.0-20180924233533-871d90846fc3
)

and my go file looks like this:

package main
import (
	docopt "github.com/caldwell/docopt.go"
)

[artburkart]

@caldwell - I believe golint will complain about this error message because the first letter is capitalized.
https://github.com/golang/go/wiki/CodeReviewComments#error-strings