Only 2048bit cert will work

[FrNico]

Either update inline comment saying: "2) Assumes you already have a valid private key, signed 2048bit certificate, ..."
Or eventually test certificate and stop, showing an error if provided cert is not 2048bit.

I was trying with a 4096bit cert, all was working fine, but unifi webUI was no longer reachable.

Thanks for the good work!