Include additional proposed change information for certain kinds of planning errors

[apparentlymart]

Some time ago we ruled that Terraform Core would make a best effort to return a partial plan describing the subset of actions that were successfully proposed before encountering an error, which Terraform CLI and Terraform Cloud then rely on to present some additional context to support the associated error messages.

This change aims to improve that effort by making a distinction between the failure of the planning operation itself vs. something else in the configuration ruling that the successfully-created plan is unacceptable for some separate reason.

In that case, it's helpful to still include that proposed change in the plan, because the planning step itself succeeded and these problems are in a sense "between" the planning operations, blocking any downstream work from starting.

In particular, this change arranges for a failed prevent_destroy check or a failed postcondition check to still include the planning result that it was checked against, which then allows the UI the option of displaying that planned action alongside the error describing why it was unacceptable.

---

The CLI layer already supports this from our earlier work in this area, and so no changes are required there. The following is an example result from a prevent_destroy violation with these changes in place:

Terraform used the selected providers to generate the following execution plan.
Resource actions are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform planned the following actions, but then encountered a problem:

  # null_resource.b must be replaced
-/+ resource "null_resource" "b" {
      ~ id       = "4627279270654855411" -> (known after apply)
      ~ triggers = { # forces replacement
          ~ "foo" = "bar" -> "baz"
        }
    }

Plan: 1 to add, 0 to change, 1 to destroy.
╷
│ Error: Instance cannot be destroyed
│ 
│   on prevent-destroy-error.tf line 7:
│    7: resource "null_resource" "b" {
│ 
│ Resource null_resource.b has lifecycle.prevent_destroy set, but the plan
│ calls for this resource to be destroyed. To avoid this error and continue
│ with the plan, either disable lifecycle.prevent_destroy or reduce the scope
│ of the plan using the -target option.
╵

Combining the error message with the proposed change first explains why this resource instance was proposed to be replaced, and then explains that it's invalid to do so. This presentation could potentially be improved in future to connect these two parts of the output more strongly, but that'll be for a future PR (focused at the CLI layer, rather than the Core layer) if so.

---

Closes #34305 and closes #30271.

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