Improve exception handling in tkinter app for better user experiance

[Agent-Hellboy]

Handle below exception

./RstEyeApp
^C^C
^C^X
^CException in Tkinter callback
Traceback (most recent call last):
  File "tkinter/__init__.py", line 1967, in __call__
  File "tkinter/__init__.py", line 861, in callit
  File "app.py", line 147, in show_image
  File "tkinter/__init__.py", line 1504, in mainloop
KeyboardInterrupt
^C^C^C
^C
^CTraceback (most recent call last):
  File "app.py", line 166, in <module>
  File "app.py", line 160, in start
  File "tkinter/__init__.py", line 1504, in mainloop
KeyboardInterrupt
[669410] Failed to execute script 'app' due to unhandled exception!