valset: better error message when jail lock not expired

[ueco-jb]

🤷 https://github.com/confio/poe-contracts/blob/main/contracts/tgrade-valset/src/contract.rs#L276
Copied from confio/tgrade-contracts#437

[ethanfrey]

Let's back port to 0.5.x before closing

(both on main and 0.5.x branch)

[ueco-jb]

@ethanfrey done.