[type/__download] bugfix checksum not found

when the checksum is not found while using __download it displays and error
saying "no checksum from target" this information is wrong and confusing. In
fact, the state explorer is missing a default condition, when the other
conditions are not satisfied

cdist/conf/type/__download/explorer/state

@@ -61,6 +61,9 @@ else
                 sum_is="sha256:$( sha256sum "$dst" | awk '{print $1}' )"
             ;;
         esac
+    else
+        echo "could not detect checksum method, check type's manual" >&2
+        exit 1
     fi
 fi