Add to behat an after step check to see if there is an error call stack

[Robert Lyon]

Currently we can do
And I should not see "Call stack"

But it would be better to have some fast way of checking after every step if a callstack error has been generated

But rather than check the DOM (which will be slow) we could have the lib/error.php set a session flag to check for.

Also we might not want to throw behat error / make it a failed step but just alert us to the fact that backtrace has been generated so we can check if it's valid or not