introduce custom UsageError

This is a exception to be thrown when the error occures because
of bad user data. We don't want to print a full stack trace in
these cases but just tell the user what went wrong.

nominatim/db/connection.py

@@ -27,7 +27,7 @@ class _Cursor(psycopg2.extras.DictCursor):
         self.execute(sql, args)
 
         if self.rowcount != 1:
-            raise ValueError("Query did not return a single row.")
+            raise RuntimeError("Query did not return a single row.")
 
         return self.fetchone()[0]