Proper PDO connectione errors

Summary: Use the error code and message returned from the PDO driver instead of
using a generic error message when a PDO connection fails.

Closes #3467.
Closes https://github.com/facebook/hhvm/pull/4053

Reviewed By: @ptarjan

Differential Revision: D1637717

Signature: t1:1637717:1414620461:1a83489e9ee93cafb516de614ee271e19b1e955b