Improve error message of missing tlp

* #34

cmd/csaf_provider/actions.go

@@ -172,7 +172,7 @@ func (c *controller) upload(r *http.Request) (interface{}, error) {
 	// Extract real TLP from document.
 	if t == tlpCSAF {
 		if t = tlp(strings.ToLower(ex.tlpLabel)); !t.valid() || t == tlpCSAF {
-			return nil, fmt.Errorf("not a valid TL: %s", ex.tlpLabel)
+			return nil, fmt.Errorf("TLP label missing in the document")
 		}
 	}