Should exit with non-zero code when scanning fails

Describe the bug

When bandit is unable to scan one or more files (e.g. due to Python 3.14 as in #1219 or non-UTF-8 characters as in #882) it still exits with exit code 0, which is misleading and makes it more difficult to check the result programmatically (e.g. to fail a build in CI systems).

Reproduction steps

1. Run bandit using Python 3.14 in any project.
2. Observe that the "Files skipped" section of the output lists all .py files with "(exception while scanning file)" and the exit code is 0.

Expected behavior

bandit would exit with a non-zero exit code to indicate failure. (Perhaps code 2 to distinguish internal errors from failed tests which set exit code 1?)

Bandit version

1.8.3 (Default)

Python version

3.13 (Default)

Additional context

No response

Start by tracing how Bandit reports files skipped with “exception while scanning file” and how the CLI selects its exit status. Reproduce the Python 3.14 or non-UTF-8 case, then add coverage for a non-zero status while preserving the existing status for failed tests. Done means scan failures are distinguishable programmatically.