broadcast E_STOP error after serial reconnection

[fabiansch]

in following case machine 2 will work normally even if on machine 1 an E_STOP button is pushed:

• disconnect serial cable that is plugged in COM2 of machine 2
• push E_STOP on machine 1
• plug serial cable again in COM2 (left one) of machine 2

When serial connection is again established machine 2 will run again.

[fabiansch]

working in broadcast_e_stop branch