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>
5ce420ac84