Try to parse output as json in case of an error exit code, and output the error key

check_iperf3.py

@@ -150,6 +150,12 @@ def run_iperf3(options) -> Union[dict, NoReturn]:
         if e.errno == 2:
             nagexit(3, "cannot find iperf3")
     except subprocess.CalledProcessError as e:
+        try:
+            json_data = json.loads(e.output)
+            if "error" in json_data:
+                nagexit(3, json_data["error"])
+        except Exception as ex:
+            pass
         nagexit(3, e.output)
     nagexit(3, "what happended?")