A pipe in query text outside of entity label produces cryptic index error

[serapio]

If you have a query entity marked up with a missing open brace, the markup parse ends up with an IndexError and little helpful information.
Example query:

book from now|sys_time} until kingdom come

Output:

...
  File "python3.6/site-packages/mindmeld/markup.py", line 81, in load_query
    markup, query_factory=query_factory, query_options=query_options
  File "python3.6/site-packages/mindmeld/markup.py", line 261, in process_markup
    raw_text, annotations = _parse_tokens(_tokenize_markup(markup))
  File "python3.6/site-packages/mindmeld/markup.py", line 398, in _parse_tokens
    annotation = stack[-1]

We should be catching this error in process_markup and making it a MarkupError with msg like

        msg = "Invalid markup in query {!r}: {}"