Improve error when non-upgradable HTTP request is sent to _blipsync endpoint

When a non-upgradable plain HTTP request is sent to /_blipsync, a request panic occurs, and it's not exactly clear to the user what the reason is.

An easy way to trigger this, is to visit the /_blipsync endpoint via a web browser. But the issue can also be seen via CBL 2.x if connecting through a non-websocket compatible proxy.

This is inherent to the Go websocket implementation, but it can be caught earlier in handleBlipsync in order to respond with a more sensible status code and error message.

https://github.com/couchbase/sync_gateway/issues/4651