rc formatter: Make sure the formatter returned successfully

The previous implementation used to replace the contents of the buffer with
whatever the `formatcmd` was returning, regardless of the exit code of the
command, which led to the buffer being wiped out on error.

This commit does the formatting in a temporary file, and only replaces the
current buffer with the contents of the -formatted- temporary file if the
`formatcmd` returned successfully.

Fixes #1357

0 files changed, 0 insertions, 0 deletions