Fix 'Undefined variable' PHP notice in API REST

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.

Remove $t_show_detailed_errors variable as it is no longer needed.

Regression introduced by b2119ce0dc944f1db13813e863471e2bc3ae64f0.

Fixes 0025429