SetGlobals: Fix segfault on invalid input (#6321)

Author: khitrin

src/passes/SetGlobals.cpp

@@ -41,7 +41,7 @@ struct SetGlobals : public Pass {
       auto value = nameAndValue[1];
       auto* glob = module->getGlobalOrNull(name);
       if (!glob) {
-        std::cerr << "warning: could not find global: " << name << '\n';
+        Fatal() << "Could not find global: " << name;
       }
       // Parse the input.
       Literal lit;