Issue Labeler - Description Textbox: Display over-limit prompt: "exceeds by N characters" when the description exceeds 100 characters.

# Description

Currently, GitHub Issue labeler shows a generic error for overlong descriptions:

`Description is too long (maximum is 100 characters)` (see screenshot below).

This increases the time users spend editing the **Description** field in cases of overlong description because they cannot see how many characters to remove without using an external character counter tool.

# Suggested Solution

* Prompt the minimum number of characters to be reduced. For instance, " Trim the message by 17 characters (limit: 100)".
or
* Prompt the number of characters over the limit. For instance, "Over by 17 characters (limit:100)".

# Alternate Solution

* Continue with current sub-optimal error prompt which requires users to use an external tool to determine minimum number of characters to trim.

# Code to demonstrate issue

# Checklist for Completion

* [ ] Add the logic to compute `excess - max(0, input_length - 100)` when input > 100
* [ ] Customize the error prompter to prompt one of the suggested solutions
* [ ] Add unit / integration tests for boundary / edge cases (99 / 100 / 101 characters)
* [ ] Update relevant documentation for the Description field error behavior