Signup - Validation messages do not display label text

When the user encounters an error on the signup page (runestone/default/user/register), the validation messages include the variable names for the fields with the errors (first_name, course_id) instead of the label text (First Name, Course Name).

To recreate:

1. Go to the Runestone Academy home page.
2. In the main navigation, click Sign Up.
3. Signup form displays. Click the terms and conditions checkbox to check it.
4. Click the Sign Up button.

Errors display:

username: Value already in database or empty
first_name: Cannot be empty
last_name: Cannot be empty
email: Invalid email
password: Too short
course_id: Unknown course name. Please see your instructor.```

Please display the label text for each (including the proper capitalization) instead of the variable name.

![Image](https://github.com/user-attachments/assets/9f910bd5-60dc-4e98-a1d9-526dd93e3230)

Start at the signup page entry point, runestone/default/user/register, and reproduce the validation errors described in the issue. Trace how the signup form builds its error messages and update the displayed field names to the corresponding capitalized labels. Done means the errors show label text such as “First Name” and “Course Name” rather than variable names.