- close loophole through which one could sneak in illegal property names

- close loophole through which one could sneak in illegal property names
  containing slashes, colons and all sorts of evil characters. In Nasal
  this could be done via props.globals.getChild("1!@#$//[]{}", 0, 1).setValue(0);
  The cause is that getChild() hands the given name directly over to an
  alternative SGPropertyNode ("convenience") constructor which sets the
  name without any checks.
- unify exception messages: first character is lower case