e2e: show command output on failure

When a command fails, it's nice to have the full output, as it contains
diagnostic information. The status code isn't sufficient for debugging.

e2e/e2eutil/cli.go

@@ -10,8 +10,12 @@ import (
 // Command sends a command line argument to Nomad and returns the unbuffered
 // stdout as a string (or, if there's an error, the stderr)
 func Command(cmd string, args ...string) (string, error) {
-	out, err := exec.Command(cmd, args...).CombinedOutput()
-	return string(out), err
+	bytes, err := exec.Command(cmd, args...).CombinedOutput()
+	out := string(bytes)
+	if err != nil {
+		return out, fmt.Errorf("command %v %v failed: %v\nOutput: %v", cmd, args, err, out)
+	}
+	return out, err
 }
 
 // GetField returns the value of an output field (ex. the "Submit Date" field