Follow-up to #2638140: Error logging should log a backtrace consistently
Problem/Motivation
Software has bugs, so errors are there and will be logged.
Sadly by default Drupal just puts the line of the broken code, which is already helpful, but on the other hand often now really helpful, because you need a full backtrace to figure something out.
Proposed resolution
Log a backtrace on top of the actual error message.
This will just make the backtrace available in the variables section, but won't be displayed as part of the watchdog site.
D7: Make the new setting opt-in and provide a way to get the value.