SimpleFilterParser - better reporting

[douglm]

For example: this expression
(vpath="/user/agrp_calsuite-MainCampus/Ongoing") and (entity_type="event"|entity_type="todo")
results in a null filter if that path is unavailable.

This code should throw an error if it tries to return null (note the caller does but we don't see the context)

if (pi.equals(PropertyInfoIndex.VPATH)) {
  checkSub(pis, 1);
  // expect list of virtual paths.
  final ArrayList<String> vpaths = doWordList();

  for (final String vpath: vpaths) {
    final FilterBase vpf = resolveVpath(vpath);

    if (vpf == null) {
      continue;
    }

    filter = and(filter, vpf);
  }

  return filter;
}