rust: fix rust-analyzer testing

[tsachiherman]

What

Following a recent dependency update, the rust-analyzer started failing when executing the following:

rustup run nightly rust-analyzer analysis-stats . 2>&1 | (! grep ERROR)

Why

One of the dependencies was including the text "ERROR", which caused this test to fail. This is a false-positive,
since there is no actual failure here. The solution here is to omit the stdout and use the stderr only when scanning for the "ERROR" text.

Known limitations

N/A

requested change applied