fix(rds): incorrect error message for rds proxies (#29404)

Closes #29402.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: msambol

packages/aws-cdk-lib/aws-rds/lib/proxy.ts

@@ -104,7 +104,8 @@ export class ProxyTarget {
 
     const engineFamily = engine.engineFamily;
     if (!engineFamily) {
-      throw new Error(`Engine '${engineDescription(engine)}' does not support proxies`);
+      throw new Error('RDS proxies require an engine family to be specified on the database cluster or instance. ' +
+        `No family specified for engine '${engineDescription(engine)}'`);
     }
 
     // allow connecting to the Cluster/Instance from the Proxy

packages/aws-cdk-lib/aws-rds/test/proxy.test.ts

@@ -203,7 +203,7 @@ describe('proxy', () => {
         vpc,
         secrets: [new secretsmanager.Secret(stack, 'Secret')],
       });
-    }).toThrow(/Engine 'mariadb-10\.0\.24' does not support proxies/);
+    }).toThrow(/RDS proxies require an engine family to be specified on the database cluster or instance. No family specified for engine 'mariadb-10\.0\.24'/);
   });
 
   test('correctly creates a proxy for an imported Cluster if its engine is known', () => {