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

issue #10 is fixed.
I wasn't sure whether this was worth bringing up, given that whoever takes #28 will ostensibly be responsible for fixing our inconsistent messages, but the whole reason #28 exists at all is because we've ignored error messages for a while now, so…

Have you read the PostgreSQL documentation on [_Reporting Errors Within The Server_][error reporting] and its [_Error Message Style Guide_][error style]? They provide good detail on what kinds of messages, syntax, and even punctuation we should be using in here…

[error reporting]: http://www.postgresql.org/docs/9.3/static/error-message-reporting.html
[error style]: http://www.postgresql.org/docs/9.3/static/error-style-guide.html