Provide more detailed explanations when a custom URL string is invalid

[cjhenck]

As seen in issue #69, the error message in the Custom URL dialog doesn't necessarily provide detailed information that can be used to fix an invalid URL. Specifically, it should be easy to notify users if a URL is missing a valid path.

[travis-mcginley]

Hello,

I'd like to take a stab at this, if that's ok?

[bemasc]

It's yours!

[crew4651]

Программа интра впн не работает что делать