No proper error message if resources are missing

[dominique-unruh]

libisabelle/modules/setup/src/main/scala/Resources.scala

Line 97 in ea0649b

val in = classLoader.getResourceAsStream(s".libisabelle/$file")

After this line, I think it would be good to check if in==null, and, if so, output an error (this happens if the resource does not exist). That should make it easier to track down errors w.r.t missing resources.