Fix exit code for invalid scan targets (#1326)

[soh2970]

Fixes #1326

Bandit previously returned exit code 0 when scan targets were invalid (ie. nonexistent directories), even though files were skipped due to errors. This could cause silent failures in CI/CD pipelines.

This change updates the CLI exit logic to return a non zero exit code when scan targets are invalid.

Tests were updated to reflect the corrected behavior, and a functional test was adjusted to expect a non zero exit code for nonexistent paths.

Behavior after fix:

• Invalid scan targets → exit 1
• Security findings → exit 1 (unless --exit-zero is set)
• Clean scan → exit 0