[consutil] Fix issue where the show line command crash if no ttyUSB exists

[Blueve]

- What I did

Address this issue: #1172
The show line command crashed with ls command error if current device doesn't have ttyUSB device under \dev

- How I did it

Reverse the control of run_command function which enable the function user can decide if exit current context.

- How to verify it

Build python-wheel package and tested it on a physical DUT.

- Previous command output (if the output of a command-line utility has changed)

Command resulted in error: ls: cannot access '/dev/ttyUSB*': No such file or directory

- New command output (if the output of a command-line utility has changed)

  Line    Baud    PID    Start Time    Device
------  ------  -----  ------------  --------
     1  115000                        switch1
     2    9600                              -