Add option to fail an audit if a dependency can't be audited

[tetsuo-cpp]

Is your feature request related to a problem? Please describe.

If an environment contains unauditable dependencies, some users might prefer that pip-audit fails loudly by returning a non-zero exit code instead of just logging a warning and skipping it.

Describe the solution you'd like

Provide some kind of command line flag (maybe --strict) to control whether to fail the audit when a dependency can't be audited.

[woodruffw]

I think this will also be solved nicely by the approach I braindumped in #141 (comment) -- when --strict is passed, we can fail with a reasonable error message on the first SkippedDependency we encounter.