[Reflection] Add lightweight error handling to ReflectionContext
Reflection metadata lookup failures are notoriously difficult to debug
because there is no error handling in TypeLowering outside of
compile-time #ifdef'd fprintf(stderr) calls. The nicest thing to do
would be to adopt llvm::Expected<> but TypeLowering is also included
in the standard library, which only has access to a tiny subset of the
LLVM Support library. This patch adds a place to store a pointer to
the first encountered error, which can then be converted to an
llvm::Error at the API level.
