stdout / stderr policy

Currently rustfmt emits a lot of output, including warnings, Warnings, internal errors, etc.

Most of it seems to be information relevant for debugging rustfmt, but not that relevant to users which cannot do much about it in most cases.

It would be great to clean some of this up such that:

• the warnings emitted are things that users can do something about (e.g. a configuration option that doesn't exist)
• the errors emitted are errors that make rustfmt fail such that no formatting is applied (e.g. parsing errors, configuration option with incorrect value)

For all other things rustfmt could have a --verbose option (or cargo fmt -vv) that prints what are currently internal errors, exceeded max line limit, and other things that while useful for debugging, aren't really actionable for users.

No files or tests are named. Start by tracing rustfmt’s current stdout/stderr emission paths and classify which warnings and errors are actionable versus diagnostic; define and test the user-facing behavior, including a verbose mode for non-actionable output.