Validate before calculating checksum, move private functions at end, add context to mockvm errors. (backport #5157)

[mergify]

This is an automatic backport of pull request #5157 done by Mergify.
Cherry-pick of 4bb8edd has failed:

On branch mergify/bp/08-wasm/release/v0.1.x+ibc-go-v7.3.x-wasmvm-v1.5.x/pr-5157
Your branch is up to date with 'origin/08-wasm/release/v0.1.x+ibc-go-v7.3.x-wasmvm-v1.5.x'.

You are currently cherry-picking commit 4bb8edd9.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   modules/light-clients/08-wasm/keeper/keeper.go
	modified:   modules/light-clients/08-wasm/keeper/msg_server_test.go
	modified:   modules/light-clients/08-wasm/testing/mock_engine.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   modules/light-clients/08-wasm/types/vm.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[chatton]

LGTM