Use ProgramDefErr.toString to get file and line info on exit

Signed-off-by: Stefan Marr <git@stefan-marr.de>

src/som/vm/ObjectSystem.java

@@ -108,7 +108,7 @@ public MixinDefinition loadModule(final Source source) throws IOException {
       loadedModules.put(uri, module);
       return module;
     } catch (ProgramDefinitionError e) {
-      VM.errorExit(e.getMessage());
+      VM.errorExit(e.toString());
       throw new IOException(e);
     }
   }