Don't segfault when encountering permission denied errors

[penguwin]

Fixes #189

The bug was caused by accessing a nil value from ArchiveResult<Archive, Error> in order to get the Archive.Path when encountering errors during file collection (such as permission denied) for snapshot creation in snapshot.go.

I decided to always include a set Archive when sending a ArchiveResult into the channel in scanner.go. This is for the following reasons:

• We need it to report to the user which file path exactly encountered issues during snapshot creation.
• We don't want to return any other error value than nil or filepath/fs.SkipDir in our WalkFunk implementation for filepath.Walk as this will cause Walk to stop walking the entire tree. The behavior when to stop is defined via the --pedantic flag and set off by returning non-nil Error values in ArchiveResult.

This PR also fixes a another segfault when entering invalid exclude patterns such as -x "\\". knoxite will now immediately exit regardless of --pedantic with a message:

Failed to store '/tmp/knoxite-189/foo': Invalid exclude filter '\': syntax error in pattern

as the pattern will never match and is probably an unintended error/typo from the user.

[craftamap]

what about those comments?

[penguwin]

Well.. 😅 I left them there on purpose to make it clear that we would also return nil for these error-cases. Can ofc remove them if desired