driver/power/mfi_mpower: fix ExecutionError message

The message should have been an f-string to contain useful
information.

Fixes: 6980c6f ("labgrid/driver/power: Backend for Ubiquity mFi mPower")
Signed-off-by: Bastian Krause <bst@pengutronix.de>
(cherry picked from commit 005b2b2)

Author: Bastian-Krause

labgrid/driver/power/mfi_mpower.py

@@ -91,7 +91,7 @@ def power_get(host, port, index):
         elif port['output'] == 1:
             return True
         else:
-            raise ExecutionError("unexpected port output value: '{port['output']}'")
+            raise ExecutionError(f"unexpected port output value: '{port['output']}'")
     else:
         raise ExecutionError(f"unexpected http response: code {r.status_code}, content type '{r.headers['Content-Type']}' and content: '{r.text}'")