Change Path::getStatusInfo to return a boolean and error string on an error

Change Path::getStatusInfo to return a boolean and error string on an error
instead of throwing an exception.  This reduces the amount of code that is
exposed to exceptions (e.g. FileUtilities), though it is clearly only one step
along the way.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@29395 91177308-0d34-0410-b5e6-96231b3b80d8