Improve dialog message of checkEncoders

[130s]

1. By default robot.checkEncoders pops up a dialog that says:

   '!! Robot Motion Warning !!
   'Turn Relay ON.'
   'Then Push [OK]'
   

   A user wonders if this appears because of system error. Better mention in the same dialog that this is a normal behavior.

2. The robot of the same user did not start calibration even after the user selects "OK" in the dialog, which later turned out there was a hardware issue. Adding a message to indicate a potential hardware issue would fasten the diagnosis.