Decline COE - Other reason message too long

[CarlyCotton]

User Story
As an institution user, when I decline a COE and choose the "Other" option I am prompted to enter a message in a large text field. If the message I submit is greater than 500 characters (the field limit in the db), I receive an unexpected error message and the COE is not actioned. I'd like better messaging on the error message and a limit on the text field to support me so I can be more efficient.

Acceptance Criteria

• Update "Other" to "Other (500 character max)"
• Warn the user with a validation-style error message in the COE form if they enter >500 characters in the text field and try to Decline enrolment now
• If the user submits a COE decline with more than 500 characters, provide a more targeted error message instead of the unexpected error:

  "Error: The reason provided is longer than 500 characters. Please try again."