direct user to download Keplr extension

# Problem to be Solved

When someone goes to psm.inter.trade they don't get any error feedback about not having Keplr installed.

# Design

if they click "Connect Keplr" and there is no `window.keplr` then tell user how to install it.

(https://github.com/Agoric/dapp-psm/pull/29 was a different approach but would have problems with document load timing)