fix: d_gcp_auth_backend_role: error on nonexistent role

[benashz]

Ensure that an error is reported whenever the gcp-auth-backend-role data source does not exist in Vault.

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Release note for CHANGELOG:

bug: data/gcp_auth_backend_role Report an error when attempting to access a nonexistent Vault role.

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'
make dev testacc TESTARGS='-run=TestAccGCPAuthBackendRoleDataSource -v' ; TZ=UTC date 
==> Checking that code complies with gofmt requirements...
go build -o terraform-provider-vault
mv terraform-provider-vault ~/.terraform.d/plugins/
TF_ACC=1 go test $(go list ./...) -v -run=TestAccGCPAuthBackendRoleDataSource -v -timeout 20m

[...]

=== RUN   TestAccGCPAuthBackendRoleDataSource_basic
--- PASS: TestAccGCPAuthBackendRoleDataSource_basic (4.92s)
=== RUN   TestAccGCPAuthBackendRoleDataSource_gce
--- PASS: TestAccGCPAuthBackendRoleDataSource_gce (3.97s)
=== RUN   TestAccGCPAuthBackendRoleDataSource_none
--- PASS: TestAccGCPAuthBackendRoleDataSource_none (0.49s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     10.111s
Fri Oct  1 19:27:52 UTC 2021