[flang][cuda][NFC] Add some diagnostic when module or fct are not found (#119277)

Author: clementval

flang/lib/Optimizer/Dialect/CUF/CUFToLLVMIRTranslation.cpp

@@ -66,8 +66,12 @@ LogicalResult registerKernel(cuf::RegisterKernelOp op,
       llvm::FunctionType::get(
           ptrTy, ArrayRef<llvm::Type *>({ptrTy, ptrTy, ptrTy}), false));
   llvm::Value *modulePtr = moduleTranslation.lookupValue(op.getModulePtr());
+  if (!modulePtr)
+    return op.emitError() << "Couldn't find the module ptr";
   llvm::Function *fctSym =
       moduleTranslation.lookupFunction(op.getKernelName().str());
+  if (!fctSym)
+    return op.emitError() << "Couldn't find kernel name symbol";
   builder.CreateCall(fct, {modulePtr, fctSym,
                            getOrCreateFunctionName(
                                module, builder, op.getKernelModuleName().str(),