If you attempt to validate a URL at the plugin manager's proxy field and enter a malformed URL, like missing the protocol type, there's no form validation, but the typical "Oops!" banner.
Unless you know exactly what you did wrong, the page is not useful at all.
There's already existing form validation, if you miss the "www" part of the URL, but not if you miss the protocol part, like https/http/etc.
It would make sense to use proper form validation for the protocol part too, rather than just failing fatally.
Stacktrace: https://gist.githubusercontent.com/NotMyFault/30eafe1e07ca674750f6cd9186883bd2/raw/35180434c026ff3d0bdd3b052b42ead68ce13002/gistfile1.txt
Minor 
Alexander Brandes
created issue -
Basil Crow
made changes -
This would be trivial to fix by catching the IllegalArgumentException and returning FormValidation.error(e, Messages.ProxyConfiguration_MalformedTestUrl(testUrl)).