Change the verifier to never throw an exception. Instead verifyModule canoptionally...

Change the verifier to never throw an exception.  Instead verifyModule canoptionally return the string error, which is an easier api for clients touse anyway.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29017 91177308-0d34-0410-b5e6-96231b3b80d8