Swallow expected `rustfmt` errors

My workflow in Visual Studio Code + Rust Analyzer has become:

1. Make a change to Rust source code using all the analysis magic

2. Save the file to trigger `cargo watch`. I have format on save enabled
   for all file types so this also runs `rustfmt`

3. Fix any diagnostics that `cargo watch` finds

Unfortunately if the Rust source has any syntax errors the act of saving
will pop up a scary "command has failed" message and will switch to the
"Output" tab to show the `rustfmt` error and exit code.

I did a quick survey of what other Language Servers do in this case.
Both the JSON and TypeScript servers will swallow the error and return
success. This is consistent with how I remember my workflow in those
languages. The syntax error will show up as a diagnostic so it should
be clear why the file isn't formatting.

I checked the `rustfmt` source code and while it does distinguish "parse
errors" from "operational errors" internally they both result in exit
status of 1. However, more catastrophic errors (missing `rustfmt`,
SIGSEGV, etc) will return 127+ error codes which we can distinguish from
a normal failure.

This changes our handler to log an info message and feign success if
`rustfmt` exits with status 1.

Another option I considered was only swallowing the error if the
formatting request came from format-on-save. However, the Language
Server Protocol doesn't seem to distinguish those cases.

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