Validate the mastodon push forms status character count

Problem/Motivation

Mastodon instances only accept a certain number of characters, and the system has the facility to show how many characters there are in the post. The issue is that the push forms do not validate the input and users will instead see an error when attempting to push a string that is too long for the mastodon instance. The error will flush the form, losing the current form values, and cause user frustration.

Steps to reproduce

Install the module and configure at least one mastodon instance.
Set the character limit on the mastodon instance (or self configure the character limit).
Head to the push status form.
See the configured character limit under the text box.
Add more text than the character limit and submit.
If the mastodon instance character limit is exceeded then the form will return an error.

This applies to both the single push status form and the entity form.

Proposed resolution

Instead of returning an error the form should attempt to validate against the same character limit.