Some multitest cleanup

[ethanfrey]

Some issues arose when debuging multitests with tgrademsg.

• We use Unauthorised everywhere, which is less than helpful when many contracts are involved. Let's make them more explicit. Starting with the TgradeMsg bindings - this should return something more clear, like "Missing Privilege". We can also consider using different terms for other contracts as well.
• We need to better document how to promote contracts at the beginning of multitest so they can make privileged actions.
• The comments on GovProposal::Migrate should be more clear
• We should implement GovProposal::Instantiate (just like WasmMsg::Instantiate) and GovProposal::Migrate (which is like WasmMsg::Migrate but overrides the admin check, so it is always allowed)

[ueco-jb]

@ethanfrey

We need to better document how to promote contracts at the beginning of multitest so they can make privileged actions.

Actually promotion is very easy:

        app.promote(owner.as_str(), contract.as_str()).unwrap();

It's just important to know (or remember) to do it. Is it still worth documenting? Where, if so?

The comments on GovProposal::Migrate should be more clear

This needs some more explanation. I agree that existing "comments" provides little to no information:
https://github.com/confio/tgrade-contracts/blob/main/packages/bindings/src/gov.rs#L58
but what exactly would you see improved there?

implement GovProposal::Instantiate (just like WasmMsg::Instantiate)

What would be its purpose? WasmMsg::Instantiate instantiates a new contract from previously uploaded wasm code. There's already similar GovProposal::InstantiateContract message.
Actually same question for migrate version.

[ethanfrey]

The point was that sometimes you do not have permission to do so (someone else's contract... in wasmd it is possible to upload code but not allow others to instantiate it)

The GovProposal variants lets you override any such permissions (eg. migrate Bart's contract), but only with approval of chain governance