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

issue #10 is fixed.

Review tasks:

  - [x] Ensure SQL keywords are capitalized in error text
  - [x] Quote identifiers, variable names, file names, parameter values etc. using double quotes
  - [x] Get rid of the word "execute" in the message (we're not to execution yet)
  - [x] Add an `errdetail` to explain why the query could not be planned
  - [x] Clarify the hint text