f2scan: improve the error message when 2 motors are using the same channel id

Feedback from ID15 during help desk.

The f2scan with 2 motors was not working.

This 2 motors was part of the same icepap switch, connected to the same musst channel.

As result, the scan was not able to do anything, but the error message was criptic.

    self.musst = self.musst_list.find_musst_for_motors(pars.motor, pars.slave_motor)
  File "/users/blissadm/local/fscan.git/fscan/mussttools.py", line 36, in find_musst_for_motors
    f"Cannot find all motors {motor_names} on one musst {self.musst_names} !!"
ValueError: Cannot find all motors ['hrx', 'mcrz'] on one musst ['musst_eh3'] !!

It would be nice to provide a more explicit message in this case.

Maybe something like "motor1 and motor2 can't be part of the same scan cause their encoders are connected to the same Musst channel id".