Description:
------------
Affects at least PgSQL driver (MySQL works fine). 

Workflow:
1. Make invalid query - errorInfo()[2] contains error info. Error code is not 
zero.
2. Make valid query - errorInfo()[2] still contains an error from the first step 
(but error code is 0).

pdo_pgsql:
libpq version 9.0.8
Module version: 1.0.2


Test script:
---------------
$pdo->exec("SELECT * FROM bad_table;");
echo $pdo->errorInfo()[2]; // "relation bad_table doesn't exist"

$pdo->exec("SELECT * FROM good_table;");
echo $pdo->errorInfo()[2]; // "relation bad_table doesn't exist". Expected "".

Expected result:
----------------
""

Actual result:
--------------
"relation bad_table doesn't exist"