fix(bundler): fail when the configured bundler package is missing

An unresolvable bundler package fell through to plain webpack, which
does not understand the arguments the CLI passes, so the run failed
with an unknown-option error naming neither the package nor the
setting that selected it.

Author: edusperoni

lib/services/bundler/bundler-compiler-service.ts

@@ -1108,6 +1108,19 @@ export class BundlerCompilerService
 			}
 		}
 
+		// Reaching here means the configured package could not be resolved.
+		// Falling through to plain webpack would run a bundler that does not
+		// understand the arguments the CLI passes, so the failure is reported
+		// against the package the project actually asked for.
+		const bundlerPackageName = this.getBundlerPackageName();
+		if (bundlerPackageName !== WEBPACK_PLUGIN_NAME) {
+			this.$errors.fail(
+				`Unable to resolve '${bundlerPackageName}'. Install it in the ` +
+					`project, or remove the bundler configuration to use ` +
+					`${WEBPACK_PLUGIN_NAME}.`,
+			);
+		}
+
 		const packagePath = resolvePackagePath("webpack", {
 			paths: [projectData.projectDir],
 		});