[MCJIT] Check for RuntimeDyld errors in MCJIT::finalizeLoadedModules.

Patch based on https://reviews.llvm.org/D75912 by Alexander Shishkin. Thanks
Alexander!

To minimize disruption to existing clients, who may be relying on the fact that
unused references to unresolved symbols do not generate an error, this patch
makes error checking opt-in: Clients can call ExecutionEngine::hasError or
LLVMExecutionEngineGetError to check whether and error has occurred.

Differential revision: https://reviews.llvm.org/D75912