Decline COE - Other reason message too long

**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 50 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 (150 character max)"
- [ ] Warn the user with a validation-style error message in the COE form if they enter >150 characters in the text field and try to Decline enrolment now
- [ ] If the user submits a COE decline with more than 150 characters, provide a more targeted error message instead of the unexpected error:
> "Error: The reason provided is longer than 150 characters. Please try again."

Start from the COE decline form and its submission path described in the issue; locate the "Other" label, the reason text field, and the handling for an overlong decline reason. Confirm the form warns above 150 characters and that submission returns the specified targeted error instead of the unexpected error.

Written by the indexing model from the issue text.