Change `rustfmt` errors to warnings?

[bendk]

We should consider turning errors when running rustfmt on the scaffolding code into warnings. This would remove friction when rustfmt is not installed and it's how the we handle errors with bindings code formatters.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-190

[jplatte]

... or use prettyplease:

• specifically meant to format generated code
• can be used as a library on stable unlike rustfmt (IIRC)
• elimintes unneeded writing of unformatted code to disk
• ensures consistent formatting regardless of the version of rustfmt available on the system, if any