handle_formatting: Notice if rustfmt is missing and report

In an attempt to fix #6052 and #4249 this attempts to detect
if rustfmt is a rustup proxy which isn't installed, and reports
the error message to the user for them to fix.

In theory this ought to be memoised but for now it'll do as-is.

Future work might be to ask the user if they would like us to
trigger the installation (if possible).

Signed-off-by: Daniel Silverstone <dsilvers@digital-scurf.org>

0 files changed, 0 insertions, 0 deletions