Catch exceptions in apply_role_pre_chef_call

As this method can be overridden in barclamps, it can be buggy. And when
it's buggy, we don't want to block things. Until now, the nodes was
still in state applying, and this was making it completely unusable.

By catching a potential exception, we can recover.
Merge pull request #567 from vuntz/apply_role_pre_chef_call-exceptions

Catch exceptions in apply_role_pre_chef_call