Reintroduce ErrStateLocked

[daniel-ciaglia]

re-introduce ErrStateLocked as it is needed for tofu-controller

See:

• partially reverted PR: Remove exit error types hashicorp/terraform-exec#352
• required error: https://github.com/search?q=repo%3Alynqtech%2Ftofu-controller%20ErrStateLocked&type=code

Tests

$> make all

go mod tidy
go mod verify
all modules verified
go vet ./...
go build ./...
go test -cover -race github.com/hashicorp/terraform-exec/internal/version github.com/hashicorp/terraform-exec/tfexec github.com/hashicorp/terraform-exec/tfexec/internal/testutil
        github.com/hashicorp/terraform-exec/internal/version            coverage: 0.0% of statements
ok      github.com/hashicorp/terraform-exec/tfexec      121.580s        coverage: 62.7% of statements
        github.com/hashicorp/terraform-exec/tfexec/internal/testutil            coverage: 0.0% of statements
TFEXEC_E2ETEST_VERSIONS=1.5.7 go test -race -timeout=30m -v ./tfexec/internal/e2etest
[...]
PASS
ok      github.com/hashicorp/terraform-exec/tfexec/internal/e2etest     186.426s