chore: extrapolate error handling into a reusable pattern

The only instance of error handling is currently in [`src/components/ProposalForm.tsx`](https://github.com/tulsawebdevs/website/blob/bb2f75dbe4c38b8eefed88eb2235253f2b41d4aa/src/components/ProposalForm.tsx). We should create a reusable utility that covers common types of errors but is also flexible enough for custom errors.