fix(angular): throw error when npmScope is incorrect

packages/angular/src/generators/utils/project.ts

@@ -29,5 +29,20 @@ export function normalizePrefix(
   // https://github.com/angular/angular-cli/blob/1c634cd327e5a850553b258aa2d5e6a6b2c75c65/packages/schematics/angular/component/index.ts#L130
   const htmlSelectorRegex =
     /^[a-zA-Z][.0-9a-zA-Z]*(:?-[a-zA-Z][.0-9a-zA-Z]*)*$/;
-  return npmScope && htmlSelectorRegex.test(npmScope) ? npmScope : undefined;
+  const normalizedPrefix =
+    npmScope && htmlSelectorRegex.test(npmScope) ? npmScope : undefined;
+
+  if (!normalizedPrefix) {
+    throw new Error(`The "--prefix" option was not provided, therefore attempted to use the "npmScope" defined in "nx.json" to set the application's selector prefix, but it is invalid.
+
+    There are two options that can be followed to resolve this issue:
+     - Pass a valid "--prefix" option.
+     - Update the "npmScope" in "nx.json" (Note: this can be an involved process, as other libraries and applications may need to be updated to match the new scope).
+
+    If you encountered this error when creating a new Nx Workspace, the workspace name or "npmScope" is invalid to use as the selector prefix for the application being generated.
+
+    Valid selector prefixes must start with a letter, and must contain only alphanumeric characters or dashes. When adding a dash the segment after the dash must also start with a letter.`);
+  }
+
+  return normalizedPrefix;
 }