Include rustfmt's stderr in errors returned from `rustfmt` function.

PiperOrigin-RevId: 427455092

rs_bindings_from_cc/token_stream_printer.rs

diff --git a/rs_bindings_from_cc/token_stream_printer.rs b/rs_bindings_from_cc/token_stream_printer.rs
index bec091a..37b1ce2 100644
--- a/rs_bindings_from_cc/token_stream_printer.rs
+++ b/rs_bindings_from_cc/token_stream_printer.rs
@@ -87,6 +87,7 @@
         ])
         .stdin(Stdio::piped())
         .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
         .spawn()
         .unwrap_or_else(|_| panic!("Failed to spawn rustfmt at '{}'", rustfmt));
 
@@ -97,8 +98,7 @@
     let output = child.wait_with_output().expect("Failed to read rustfmt stdout");
 
     if !output.status.success() {
-        // The rustfmt error message has already been printed to stderr.
-        bail!("Unable to format output with rustfmt");
+        bail!("rustfmt reported an error: {}", String::from_utf8_lossy(&output.stderr));
     }
 
     Ok(String::from_utf8_lossy(&output.stdout).to_string())