Add KeyboardInterrupt error handling

[sh0213]

Fixes Bug #2878.
Add a KeyboardInterrupt Interrupt Handler inside the try catch
statement of the main program. Thus if a user will use Ctrl+C
to kill the updater it will get the following message:
"Program interrupted by keyboard, aborting.".

• I have read the contributing guide lines at
  https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
• I have signed the Open Information Security Foundation
  contribution agreement at
  https://suricata-ids.org/about/contribution-agreement/
• I have updated the user guide (in doc/userguide/) to reflect the
  changes made (if applicable)

Link to redmine ticket: #2878

Describe changes:

• When a user sends a kill signal (interrupts the program) using Ctrl+C the trace isn't shown anymore but instead the informative message "Program interrupted by keyboard, aborting".
• Changes involved on the main.py by adding an Exception Handler inside the try-catch main loop.

[shivan1b]

@sh0213 I'm sorry but we'll have to close this PR since this task was already claimed and there is a PR for that. Do you mind looking at the issues here and claim one of these tasks which have the "New" status?
Please remember to follow the contribution guidelines as mentioned in the Outreachy project pages.
Thank you. :)