drupal_log_error() logs a backtrace now, Error::decodeException() return value changed

As of now we not only log the line in which a PHP error / exception happened, but we even log an entire backtrace. This makes it much easier for people to figure out where something may have gone wrong.

Since 8.0.5 the return value of \Drupal\Core\Utility\Error::decodeException() has contained an undocumented key backtrace_string. In 8.2.0 this key has been renamed @backtrace_string.