Ability to use 'show-unmatched-globs' in CI pipelines for missing specs

[axelssonHakan]

Describe the bug
Right now the show-unmatched-globs only logs a message if a spec isn't found using a glob. It would be nice if it would change the exitCode as well (process.exitCode = 1;).
https://github.com/stoplightio/spectral/blob/develop/src/cli/services/linter/linter.ts#L73

Maybe the yarg option --show-unmatched-globs could be made to take a string and use that to change exit code if that is behaviour the user wants

• --show-unmatched-globs=empty/info/warn >> exitCode=0
• --show-unmatched-globs=error>> exitCode=1

To Reproduce

1. Run this CLI command npx spectral lint --show-unmatched-globs globReturningEmptyArray
2. echo $? >> 0

Expected behaviour
In CI environments it would be preferred if the --show-unmatched-globs returned an exitCode > 0 so you don't get any "false" positive tests

[nulltoken]

@P0lip Or maybe could we add an additional switch (--fail-on-unmatched-globs)... Thoughts?

[P0lip]

My apologies for dropping a ball on this one.
--fail-on-unmatched-globs sounds good.