dotest.py: Remove useless AttributeError catches

Summary:
Test decorators were ignoring AttributeError exceptions. These were introduced three years ago,
and copied to all decorators. They seem to serve no purpose and removing them produces no errors.
Given that they have prevented us from noticing the problem in r249819, I am removing them.

Reviewers: zturner, tfiala

Subscribers: iancottrell, lldb-commits

Differential Revision: http://reviews.llvm.org/D13583

llvm-svn: 250038