correct casing of slack channel secret

.drone.yml

@@ -14,6 +14,6 @@ steps:
       SLACK_TOKEN: 
         from_secret: slack_token
       SLACK_CHANNEL:
-        from_secret: SLACK_CHANNEL
+        from_secret: slack_channel
     commands:
       - bash run.sh

bot.py

@@ -180,6 +180,9 @@ if __name__ == '__main__':
                 params={'thread_ts': msg['ts'], 'channels': channel},
                 files={'file': (filename, open(filename, 'rb').read())},
             )
+        else:
+            print('Slack API responded with error:')
+            print(msg)
 
     else:
         print('Failed to find the word.')