Student: Request a Change: "Unexpected error" displayed instead of intuitive error message when invalid data entered in Application # field

**Describe the Bug**
"Unexpected error" displayed instead of intuitive error message when Application # inputted where that App # does not correspond to an application in the student's history

**Expected Behaviour**
A clear message to enter a valid application number:
> "Please enter an application number corresponding to one of your active applications."

Ref #3033 for future dropdown list

**Actual Behaviour**
Error message 'Unexpected error. An error happened while requesting a change'

** Steps To Reproduce**

Precondition: Student user with either no application or application

Steps to reproduce the behaviour:
1. Login as a student user
2. Click on 'Request a Change'
3. Scroll down to 'Application# field'
4. Type in any data
5. Select an option from the 'Request a change to*'

**Additional context**
GDX Discovery Feedback