use multierror.Append instead of append

[alisdair]

1.5 backport of #33108. Original PR description:

---

Ran into this error while running terraform inside a container, after it applied my plans, terraform failed saving state to Consul. I suspect my policy needs tweaking, but it's near impossible to tell with an error like this:

╷
│ Error: Failed to save state
│ 
│ Error saving state: consul CAS failed with transaction errors:
│ [0xc0006e93c8]
╵

This PR changes the the rendering of the error instance to actually print the message, instead of the memory address of said error instance.

Target Release

1.5.x

Draft CHANGELOG entry

BUG FIXES

• When using consul as a state backend and failing to save state, consul CAS failed with transaction errors no longer shows an error instance memory address but an actual error message.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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