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/utils.py

@@ -32,6 +32,10 @@ reserved_words = set((
 ).split())
 
 
+class InterruptedError(Exception):
+    pass
+
+
 def urlsafe_components(token):
     "Splits token on commas and URL decodes each component"
     return [