Allow PDO drivers custom methods to trigger errors/exceptions

Moved a few definitions from php_pdo_int.h to a new php_pdo_error.h
header file that can be included if drivers need PDO's own error
handling to be triggered within custom methods (e.g. PDO::pgsqlLOBOpen).
Refactored custom PDO_pgsql methods to trigger errors/exceptions

BC Break: the custom methods were previously just return false on
failure. Now they throw an exception with a proper error message.
An hopefully welcome improvement, but some application might be
depending on the old behaviour. FWIW the PDO::pgsqlCopy* methods
are not documented, even though they are available since 5.3.x.
Fixed Bug #42614 (PDO_pgsql: add pg_get_notify support)