around can't call on constructor. then, call error method by sub BUILD

Hi. Thanks to make resque clone.

I found 2 issue to use with ruby resque-web.

1. backtrace is always undef by default.
"around 'error'" is not called on Resque::Failure->new.
So, argument error is not parsed and always backtrace is undef until set error by method call.

2. resque-web expect 'backtrace' is array.

I want to use resque-web on develpment. but resque v1.23.0 expect backtrace is array.
Then, I change backtrace isa 'Str' -> 'ArrayRef[Str]'.