Store Errors inside ClangTidyContext instead of just pointer to an external
array. This simplifies usage of ClangTidyContext a bit and seems to be more
consistent.

Reviewers: klimek

Reviewed By: klimek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D3685

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@208407 91177308-0d34-0410-b5e6-96231b3b80d8