fix(build-rs): Correctly refer to the item in assert (#14913)

Follow up to #14910

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crates/build-rs/src/output.rs

@@ -418,7 +418,7 @@ pub fn warning(message: &str) {
 #[track_caller]
 pub fn error(message: &str) {
     if message.contains('\n') {
-        panic!("cannot emit warning: message contains newline");
+        panic!("cannot emit error: message contains newline");
     }
     emit("error", message);
 }