Nil-check exception backtrace before passing it to RSpec

[lserman]

When yarjuf is passed an exception with a nil backtrace, it would hand it off the RSpec's backtrace formatter unchecked which would cause RSpec to raise a NoMethodError when calling empty?.

This PR will set formatted_backtrace to an empty string if nil, instead of passing to the RSpec backtrace formatter.

In addition, I changed the regex on the timestamp step definition to accept "+" or "-" on the UTC offset. It was failing for me since I am in CST.