Rework custom error handling

Problem/Motivation

We have custom error handling in \Drupal\rokka\RokkaStreamWrapper::triggerException(). It looks like the reason for this is mainly to output a user friendly error to the page in case of a network error: "An error occurred while communicating with the Rokka.io server!". In addition to this any exceptions are converted to user warnings.

Unfortunately this causes the exception backtraces to be lost, making it very difficult to track down bugs in the module. See for example 🐛 Fatal error "Stream must be a resource" Needs review .

Proposed resolution

Make sure we retain the backtraces and full information similar to what is output by _drupal_error_handler_real().