Omicron's Error::internal_error could be more general

[bnaecker]

This was raised by @ahl in the following context:

          maybe change `internal_error`?

Maybe these should accept impl ToString ? or AsRef<str>

Originally posted by @ahl in #4874 (comment)

I have often found myself creating an internal error by calling foo.to_string() here. That causes an allocation at the call site, passes a reference from which a new allocation is made, and then throws away the original. It might be nice to take something like impl ToString and call to_string() internally.