crash due to use_citusdb_select_logic on postgres is fixed (#10)

issue #10 is fixed.
I think the general idea is the message says what went wrong, the detail says why, and the hint suggests how to fix it.

In particular:

  * I like the `errmsg`, though `SELECT` should be uppercase and it's a bit misleading to say "execute" during planning. Maybe _could not plan SELECT query_?
  * We should probably provide a detail using `errdetail`. Maybe _Configured to use CitusDB's SELECT logic, but CitusDB is not present._
  * I don't really like the _You_ in your hint, and it only mentions one way of solving the problem when two exist. Maybe: _Install CitusDB or set the "use_citusdb_select_logic" configuration parameter to "false"._