Fail if explicitly specified config file is absent

[metametadata]

What would you like to be added:

The app should exit with error message and exit code 1 if the explicitly specified config file doesn't exist. For example, this currently doesn't fail if foo.yaml doesn't exist:

grype --config foo.yaml .

Why is this needed:

On developer mistakes the scanner silently switches to the default configuration, missing CVEs which could be found with the appropriate config (e.g. which sets add-cpes-if-none=true).

Additional context:

v0.79.3

[kzantow]

Developer note: we might need a change related to this one from @wagoodman to either return a boolean to mark something as required or return an error.