Long story short

This is a follow-up for #2479 (comment).
Currently, if one uses ClientSession and it reaches URL with custom schema the exception with insufficient metadata is being raised.

Expected behaviour

There should be some way to give control back to the caller once we reach valid URL, which we cannot follow because of unsupported schema. @asvetlov suggested to implement a custom exception for this.

Actual behaviour

ValueError: Can redirect only to http or https

Steps to reproduce

Query https://steamcommunity.com/mobileconf/conf or steammobile://lostauth/, or twitter://webknjaz