Change BitcodeReader to use error_code instead of bool + string.

In order to create an ObjectFile implementation that uses bitcode files, we
need to propagate the bitcode errors to the ObjectFile interface, so we need
to convert it to use the same error handling as ObjectFile: error_code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193996 91177308-0d34-0410-b5e6-96231b3b80d8