src: fix -Wunused-result warning

Fix a compiler warning that was introduced in commit c3dc0e0
("src: add CollectExceptionInfo & errors.SystemError") by adding
a `.FromJust()` call.

PR-URL: #16726
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

Author: bnoordhuis

src/env.cc

@@ -263,7 +263,7 @@ void CollectExceptionInfo(Environment* env,
 
   if (syscall != nullptr) {
     obj->Set(env->context(), env->syscall_string(),
-             OneByteString(env->isolate(), syscall));
+             OneByteString(env->isolate(), syscall)).FromJust();
   }
 }