Ensure sqlite_timelimit correctly clears handler

If an error occurred inside the block the progress handler (used to
enforce a time limit) was not being correctly cleared, resulting in
timeout errors potentially occurring during subsequent SQL queries.

The fix is described here: https://docs.python.org/3/library/contextlib.html#contextlib.contextmanager

datasette/utils.py

@@ -152,8 +152,10 @@ def sqlite_timelimit(conn, ms):
             return 1
 
     conn.set_progress_handler(handler, n)
-    yield
-    conn.set_progress_handler(None, n)
+    try:
+        yield
+    finally:
+        conn.set_progress_handler(None, n)
 
 
 class InvalidSql(Exception):