This PR adds a new formatError option so that consumers can customise the error messages when an error module is detected. This is useful in the context of a tool like Remix where we'll want to give our consumers more Remix-specific advice for how to fix the error since they don't have access to this esbuild plugin directly.

I've also improved the error messaging for when a module doesn't have a polyfill available. Without this we'd end up instructing our consumers to enable the polyfill, only for the polyfill to fail.

Status and versioning classification:

• Code changes have been tested and working fine, or there are no code changes
• This PR changes the library's interface (methods or parameters added)