Use ErrorOr for the ::create factory on instrumented and sample profilers.

Summary:
As discussed in
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20141027/242445.html,
the creation of reader and writer instances is better done using
ErrorOr. There are no functional changes, but several callers needed to
be adjusted.

Reviewers: bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6076