[BuildingAJIT] Update the Ch1 KaleidoscopeJIT class to expose errors to clients.

Returning the error to clients provides an opportunity to introduce readers to
the Expected and Error APIs and makes the tutorial more useful as a starting
point for a real JIT class, while only slightly complicating the code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344720 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed