db_error() doesn't work on default connection

The function db_error() doesn't work if you try to get the latest error on the default connection.

Here's some code to reproduce the issue:

<?php

include_once './includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

$result = db_query("SELECT unknown_field FROM {node}");

print "Error: ".db_error()."\n"; // db_error() always returns 0 but should return 1054 in case of MySQL

?>

The reason for this is that in case of a database error this error is reported via trigger_error(...). Drupal's error handler than calls watchdog(...) with an translated error message. Normally both SQL statements triggered by the translation using t(...) and the insert into the watchdog table don't fail. So the last SQL statement has no error and that's what db_error() returns!

A possible solution is to handle translations and watchdog entries via a dedicated connection.

Another workaround for command line scripts like cron.php is to define another error handler that doesn't trigger watchdog entries but prints out errors.