Return null on illegal field acces

issue #940

Author: mdesharnais

handlebars/src/main/java/com/github/jknack/handlebars/context/FieldValueResolver.java

@@ -212,10 +212,13 @@ public boolean matches(final FieldWrapper field, final String name) {
   protected Object invokeMember(final FieldWrapper field, final Object context) {
     try {
       return field.get(context);
+    } catch (IllegalAccessException ex) {
+      // Illegal access to field is expected for classes from external modules
+      return null;
     } catch (Exception ex) {
       throw new IllegalStateException(
-          "Shouldn't be illegal to access field '" + field.getName()
-              + "'", ex);
+        "Shouldn't be illegal to access field '" + field.getName()
+        + "'", ex);
     }
   }