update error message for instanceAlreadyExists

[megan07]

Fixes hashicorp/terraform-provider-google#6066 (comment)

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

sql: fixed bug where the provider would retry on an error if the database instance name couldn't be reused.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))
TF Conversion: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests in RECORDING mode for the following tests that failed during VCR: TestAccBigqueryDataTransferConfig|TestAccCloudBuildTrigger_pubsub_config|TestAccCloudBuildTrigger_webhook_config|TestAccComputeGlobalForwardingRule_globalForwardingRuleInternalExample|TestAccComputeGlobalForwardingRule_internalLoadBalancing|TestAccContainerNodePool_withGPU|TestAccDataLossPreventionDeidentifyTemplate_dlpDeidentifyTemplateBasicExample|TestAccDataLossPreventionDeidentifyTemplate_dlpDeidentifyTemplateUpdate|TestAccDialogflowCXFlow_dialogflowcxFlowFullExample|TestAccDialogflowCXVersion_dialogflowcxVersionFullExample|TestAccDialogflowCXFlow_update|TestAccDialogflowCXVersion_update|TestAccNetworkServicesEdgeCacheKeyset_networkServicesEdgeCacheKeysetBasicExample|TestAccNetworkServicesEdgeCacheOrigin_networkServicesEdgeCacheOriginBasicExample|TestAccNetworkServicesEdgeCacheOrigin_networkServicesEdgeCacheOriginAdvancedExample|TestAccNetworkServicesEdgeCacheOrigin_updateAndImport|TestAccNetworkServicesEdgeCacheService_networkServicesEdgeCacheServiceBasicExample|TestAccNetworkServicesEdgeCacheService_networkServicesEdgeCacheServiceAdvancedExample|TestAccNetworkServicesEdgeCacheService_updateAndImport|TestAccPrivatecaCertificateAuthority_privatecaCertificateAuthorityBasicExample|TestAccPrivatecaCertificate_privatecaCertificateCsrExample|TestAccPrivatecaCertificate_privatecaCertificateConfigExample|TestAccPrivatecaCertificate_privatecaCertificateNoAuthorityExample|TestAccSecurityCenterNotificationConfig_sccNotificationConfigBasicExample You can view the result here: https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=197249

[modular-magician]

Tests failed during RECORDING mode: TestAccDataLossPreventionDeidentifyTemplate_dlpDeidentifyTemplateBasicExample|TestAccDialogflowCXVersion_update|TestAccNetworkServicesEdgeCacheKeyset_networkServicesEdgeCacheKeysetBasicExample|TestAccNetworkServicesEdgeCacheOrigin_networkServicesEdgeCacheOriginAdvancedExample|TestAccNetworkServicesEdgeCacheOrigin_updateAndImport|TestAccNetworkServicesEdgeCacheService_networkServicesEdgeCacheServiceBasicExample|TestAccNetworkServicesEdgeCacheService_networkServicesEdgeCacheServiceAdvancedExample|TestAccNetworkServicesEdgeCacheService_updateAndImport Please fix these to complete your PR