make every log an error for testing slack (#34)

Follow-up PR to #33, which escalates the main logs to an error level,
for the purpose of testing the relevant slack channel.

src/off_chain/cow_endpoint_surplus.py

@@ -271,7 +271,7 @@ def logging_function(
         Logs to terminal (and file iff file_name is passed).
         """
 
-        self.logger.warning(
+        self.logger.error(
             "Transaction Hash: %s\nFor order: %s\nWinning Solver: %s\n"
             "More surplus Corresponding Solver: %s\nDeviation: %s\n"
             "absolute difference: %s\n",