Check if artifactdir exists early

Currently debos fails if an action attempts to use artifactdir and it
doesn't exist on the host:

  $ debos --artifactdir=out rpi64/debimage-rpi64.yaml
  Action `image-partition` failed at stage PreMachine, error: open /home/obbardc/projects/debos/debos-recipes/out/debian-rpi64.img: no such file or directory

Check that artifactdir exists early and return an error in case that it
does not exist.

Closes: #426
Signed-off-by: Christopher Obbard <chris.obbard@collabora.com>

cmd/debos/debos.go

@@ -192,6 +192,11 @@ func main() {
 		context.Artifactdir, _ = os.Getwd()
 	}
 	context.Artifactdir = debos.CleanPath(context.Artifactdir)
+	if _, err := os.Stat(context.Artifactdir); os.IsNotExist(err) {
+		log.Printf("artifactdir does not exist: %v", err)
+		exitcode = 1
+		return
+	}
 
 	// Initialise origins map
 	context.Origins = make(map[string]string)