358: Add support for formatting entire document with rustfmt r=matklad a=aleksanb
Attempting to format a document when rustfmt isn't installed will result
in an error being returned to the frontend. An alternative
implementation would be returning zero replacements.
Part of https://github.com/rust-analyzer/rust-analyzer/issues/160.
Co-authored-by: Aleksander Vognild Burkow <aleksanderburkow@gmail.com>