Fix 'Undefined variable' PHP notice in API REST

[dregad]

Since the Slim Container's 'displayErrorDetails' setting is set to true
according to $t_show_detailed_errors's value, instead of referencing the
variable directly within the closure via a 'use' statement, we retrieve
the Container's settings to determine whether PHP exception details
should be shown or not.

Regression introduced by b2119ce.

Fixes #25429 (see #1280)

[atrol]

While you are on it, you might also want to change

$t_show_detailed_errors = ON == config_get_global( 'show_detailed_errors' );
if( $t_show_detailed_errors ) {
	$t_config['settings'] = array( 'displayErrorDetails' => true );
}

to

if( ON == config_get_global( 'show_detailed_errors' ) ) {
	$t_config['settings'] = array( 'displayErrorDetails' => true );
}

[dregad]

While you are on it, you might also want to change [...]

Good suggestion, done.