fix(@angular-devkit/architect): provide better error when builder is not defined

When a builder is not defined a more actionable error message is now displayed.

Closes #29226

Author: alan-agius4

packages/angular_devkit/architect/node/node-modules-architect-host.ts

@@ -75,6 +75,9 @@ export class WorkspaceNodeModulesArchitectHost implements ArchitectHost<NodeModu
       this.workspaceHost = {
         async getBuilderName(project, target) {
           const targetDefinition = findProjectTarget(workspaceOrHost, project, target);
+          if (!targetDefinition.builder) {
+            throw new Error(`A builder is not set for target '${target}' in project '${project}'.`);
+          }
 
           return targetDefinition.builder;
         },