Adding verbose errors, removing dupe path check.

* Added verbose error messaging when a port number, etc, can't be
  parsed.
* Removed logic around duplicate path when serving files.
* Fixed CLI usage to include a port number for host addreses:
    e.g. 10.0.0.1:80
* Fixed configuration option when using 'http+https' for the proto.
    Now doing `ngrok start foobar` will correctly interpret http+https.