Make sure TableGen exits with an error code after printing errors.

This makes it possible to report multiple errors in one invocation.
There are already calls to PrintError in CodeGenDAGPatterns.cpp which
previously would not cause TableGen to fail.

<rdar://problem/13463339>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177573 91177308-0d34-0410-b5e6-96231b3b80d8