fix: Add pending & error state handling for `ProposalForm.tsx`

[helmturner]

We're currently just logging errors, but there's no indication in the UI that anything went wrong (aside from the built-in form validation stuff). There's also no indication that the form is in the process of submitting, so that would be good to add as well.

The solution for this needs to:

• Catch any errors that may occur during submission
• When an error occurs, render a component that provides a user-friendly message
• Disable the button while the form is submitting
• Indicate to the user when a submission is in progress

There aren't any existing patterns since this is the first implementation of it's kind. We'll want to extrapolate patterns used here to the project as a whole.