Show facets that timed out using new InterruptedError

If the user requests some _facet= options that do not successfully execute in
the configured facet_time_limit_ms, we now show a warning message like this:

    These facets timed out: rowid, Title

To build this I had to clean up our SQLite interrupted logic. We now raise a
custom InterruptedError exception when SQLite terminates due to exceeding a
time limit.

In implementing this I found and fixed a logic error where invalid SQL was
being generated in some cases for our faceting calculations but the resulting
sqlite3.OperationalError had been incorrectly captured and treated as a
timeout.

Refs #255
Closes #269

datasette/templates/table.html

@@ -100,6 +100,10 @@
     </p>
 {% endif %}
 
+{% if facets_timed_out %}
+    <p class="facets-timed-out">These facets timed out: {{ ", ".join(facets_timed_out) }}</p>
+{% endif %}
+
 {% if facet_results %}
     <div class="facet-results">
         {% for facet_info in sorted_facet_results %}