raise value error

app.py

@@ -28,9 +28,12 @@ def HILL(input_image, operation, message, key):
     cv2.imwrite('tmp/'+tmp_name+'.png',I)
 
     if operation == 'decode': 
-        stc.extract('tmp/'+tmp_name+'.png', key, 'tmp/'+tmp_name+'.txt')
-        return 'tmp/'+tmp_name+'.txt'
-    
+        try:
+            stc.extract('tmp/'+tmp_name+'.png', key, 'tmp/'+tmp_name+'.txt')
+            return 'tmp/'+tmp_name+'.txt'
+        except:
+            raise ValueError('Unable to decode')
+        
     else: 
         H = np.array(
            [[-1,  2, -1],