aliases: Log the "alias-exists" hook output, for debugging

The output of the alias-exists hook is unused, so currently it's
discarded silently.

However, it can be very useful to debug issues when the hook is not
working as expected.

So this patch makes chasquid log the combined output (stdout and stderr)
to the execution trace.

internal/aliases/aliases.go

@@ -453,12 +453,15 @@ func (v *Resolver) runExistsHook(addr string) bool {
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, v.ExistsHook, addr)
-	err := cmd.Run()
+
+	out, err := cmd.CombinedOutput()
+	tr.Debugf("output: %q", string(out))
 	if err != nil {
 		tr.Debugf("not exists: %v", err)
 		hookResults.Add("exists:false", 1)
 		return false
 	}
+
 	tr.Debugf("exists")
 	hookResults.Add("exists:true", 1)
 	return true