Do not silently ignore typos in --url parameter

[alekstrust]

I'm working in a multisite instance exporting and importing posts. At some point I realized the --url parameter had a missing / in the protocol and all posts were included in the primary blog.

This due to wp-cli ignoring the --url param without any notice if it's wrong.

It would be ideal to print some info of the site we are currently working on at the beginning of the output, and throw an error if the site in the given url does not exists.

[danielbachhuber]

Thanks for the report, @alekstrust !

I suppose it'd probably be helpful to include some validation for --url=<url>, as long as we didn't cause too many explosions with it.

Feel free to submit a pull request, if you'd like!

[shubham07kb]

I am taking this one.