Signal an error on recursive yas--fallback calls

* yasnippet.el (yas--fallback): let-bind yas-fallback-behavior to
  'yas--fallback before actually doing the fallback. Signal an error if
  it's 'yas--fallback (indicating we're in an infinte loop).
Merge pull request #524 from npostavs/fallback-loop

Signal an error on recursive yas--fallback calls