Support custom texts for converter errors

### What component of Red (cog, command, API) would you like to see improvements on?

Bot Core

### Describe the enhancement you're suggesting.

We've been discussing adopting a global styling for all of our messages so that any command success message starts with a ✅ and any command failure message starts with an :x:. Unfortunately, the error messages that come from the dpy errors / the Red converters are currently untouchable for this, and so there will be some discrepancy in this global policy.

It would be cool if we could overwrite the default strings with our own error message texts so that we are able to implement this on errors caught upstream from our own code.

### Anything else?

Relates to #5174 as well