Backport of Make config errors more important during init operations into v1.5

[teamterraform]

Backport

This PR is auto-generated from #33628 to be assessed for backporting due to the inclusion of the label 1.5-backport.

The below text is copied from the body of the original PR.

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This PR updates the backend initialisation logic so that if the value returned by parsing the configuration isn't wholly known it returns an error diagnostic instead of crashing. This happens because we try to delay returning diagnostics until we can validate the required_versions conditions.

This PR also tweaks the ordering of which diagnostics are returned first. Previously if the backend initialisation returned diagnostics those diagnostics would be reported instead of the config parsing diagnostics. I can see the logic in that, but there is no way to differentiate between there is an error in the backend initialisation and there was an error in the backend initialisation caused by an error in the configuration. Therefore, I think it's better to simply return the configuration errors as higher priority since in the end, both will have to be fixed anyway.

@radditude - you put together the logic I'm moving around here so would be glad to hear your thoughts on my reasoning!

Another solution could be to simply return all the early config diagnostics and the backend diagnostics together instead of either/or?

Fixes #33622

Target Release

1.5.5

Draft CHANGELOG entry

BUG FIXES

• init: Fix crash when using invalid configuration in backend blocks.

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