check: implement "missing" failure type

If a file is missing, previously the check would simply fail. This
allows you to get information even if files are missing from the tree.
This also implements some of the wrappers required to add more failure
states.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

cmd/gomtree/main.go

@@ -45,7 +45,7 @@ var formats = map[string]func(*mtree.Result) string{
 	"text": func(r *mtree.Result) string {
 		var buffer bytes.Buffer
 		for _, fail := range r.Failures {
-			fmt.Fprintf(&buffer, "modified\t%s\n", fail.Path)
+			fmt.Fprintf(&buffer, "%s\t%s\n", fail.Type(), fail.Path())
 		}
 		return buffer.String()
 	},