Fixed MODBUS slave callback when SPS30 is not connected

tests/query_device.py

@@ -121,9 +121,9 @@ if action == 'read' or action == 'all':
             reg_name = ''
         try:
             result = s.read_register(reg_number)
-        except ValueError:
-            print(f'Register number {reg_number} cannot be read (wrong number?)')
-            exit(-8)
+        except:
+            print(f'{reg_number : <10} {"N/A" : <10} Failed to read register')
+            continue
         print(f'{reg_number : <10} {int(result) : <10} {reg_name}')
 elif action == 'write':
     if len(register_name) > 0: