Fix yarn (fix #403)

src/executor.rs

@@ -236,6 +236,7 @@ impl Check for ExecutorExitStatus {
 pub trait CommandExt {
     /// Run the command, wait for it to complete, check the return code and decode the output as UTF-8.
     fn check_output(&mut self) -> Result<String>;
+    fn string_output(&mut self) -> Result<String>;
 }
 
 impl CommandExt for Command {
@@ -248,4 +249,10 @@ impl CommandExt for Command {
         }
         Ok(String::from_utf8(output.stdout)?)
     }
+
+    fn string_output(&mut self) -> Result<String> {
+        let output = self.output()?;
+        trace!("Output of {:?}: {:?}", self, output);
+        Ok(String::from_utf8(output.stdout)?)
+    }
 }