Use formatting prefix for rustfmt errors

[dertin]

Fixes#6971.

Uses Error formatting files: as the mode-independent error prefix.

[ytmimi]

From the description of #6971:

When rustfmt runs with --check and encounters a module resolution error, it prefixes the diagnostic with:

Error writing files:

This wording is misleading in check mode because --check does not write formatted output back to the source files. It determines whether formatting would produce changes.

Why is Error formatting files: better than Error writing files:?

Aren't both misleading in --check mode since we're not formatting any files either?

[ytmimi]

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[dertin]

@ytmimi That's fair. My intent was to avoid suggesting that --check attempted to write files, since the original message made me suspect a write or permissions failure in CI.

The alternatives are a mode-independent Error: (although I personally find it too generic)
or mode-specific prefixes:

• Error checking files: with --check
• Error formatting files: otherwise.

The latter requires passing the check mode into the reporting function, but it is still a small change.

Which option would you prefer?