missing directories should be cause for alarm

Describe the bug
bandit does not consider missing items to be a problem. That means that renaming a directory without adjusting the CI-scripts will take it off the radar with zero complaint. It will even "fix" all of your old security problems.

To Reproduce
tell bandit to scan a directory which does not exist.
bandit fnord
and it will tell you that everything is okay (exit-code 0).
while there is a mention of the problem, it is no cause for alarm:

Files skipped (1):
        fnord (No such file or directory)
...
 echo $?
0

Expected behavior
The test should fail with a non-zero exit-code. That will fail the CI and cause humans to look at the problem. Misconfigurations will be caught and corrected.

Bandit version

bandit 1.6.2
  python version = 3.6.9 (default, Nov  7 2019, 10:44:02) [GCC 8.3.0]

Additional context
Thanks for bandit :)