downloadOnly + DiskCacheStrategy.NONE throws exception

**Glide Version**: 4.12.0

**Integration libraries**: No

**Device/Android Version**: Emulator and Samsung Galaxy S20

**Issue details / Repro steps / Use case background**: Running the following code (Using RxJava2 for Networking, but error also was there with coroutines):

```
Single.fromCallable {
val f = Glide.with(applicationContext)
.downloadOnly()
.load("https://upload.wikimedia.org/wikipedia/commons/b/b6/Image_created_with_a_mobile_phone.png")
.submit()
.get()
}.subscribeOn(Schedulers.io()).subscribe()
```
runs with no problem. However, adding DiskCacheStrategy.NONE like so:

```
Single.fromCallable {
val f = Glide.with(applicationContext)
.downloadOnly()
.diskCacheStrategy(DiskCacheStrategy.NONE)
.load("https://upload.wikimedia.org/wikipedia/commons/b/b6/Image_created_with_a_mobile_phone.png")
.submit()
.get()
}.subscribeOn(Schedulers.io()).subscribe()
```

fails with the following `java.util.concurrent.ExecutionException` :

```
E/AndroidRuntime: FATAL EXCEPTION: RxCachedThreadScheduler-1
Process: de.nwuensche.myapplication, PID: 10684
io.reactivex.exceptions.OnErrorNotImplementedException: The exception was not handled due to missing onError handler in the subscribe() method call. Further reading: https://github.com/ReactiveX/RxJava/wiki/Error-Handling | java.util.concurrent.ExecutionException: com.bumptech.glide.load.engine.GlideException: Failed to load resource
at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:704)
at io.reactivex.internal.functions.Functions$OnErrorMissingConsumer.accept(Functions.java:701)
at io.reactivex.internal.observers.ConsumerSingleObserver.onError(ConsumerSingleObserver.java:46)
at io.reactivex.internal.operators.single.SingleSubscribeOn$SubscribeOnObserver.onError(SingleSubscribeOn.java:73)
at io.reactivex.internal.operators.single.SingleFromCallable.subscribeActual(SingleFromCallable.java:48)
at io.reactivex.Single.subscribe(Single.java:3603)
at io.reactivex.internal.operators.single.SingleSubscribeOn$SubscribeOnObserver.run(SingleSubscribeOn.java:89)
at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:578)
at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66)
at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:301)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
at java.lang.Thread.run(Thread.java:923)
Caused by: java.util.concurrent.ExecutionException: com.bumptech.glide.load.engine.GlideException: Failed to load resource
at com.bumptech.glide.request.RequestFutureTarget.doGet(RequestFutureTarget.java:217)
at com.bumptech.glide.request.RequestFutureTarget.get(RequestFutureTarget.java:130)
at de.nwuensche.myapplication.MainActivity.onCreate$lambda-1(MainActivity.kt:48)
at de.nwuensche.myapplication.MainActivity.lambda$m1NzZeOm-GuB9bgKAG9k7oILWF0(Unknown Source:0)
at de.nwuensche.myapplication.-$$Lambda$MainActivity$m1NzZeOm-GuB9bgKAG9k7oILWF0.call(Unknown Source:2)
at io.reactivex.internal.operators.single.SingleFromCallable.subscribeActual(SingleFromCallable.java:44)
at io.reactivex.Single.subscribe(Single.java:3603) 
at io.reactivex.internal.operators.single.SingleSubscribeOn$SubscribeOnObserver.run(SingleSubscribeOn.java:89) 
at io.reactivex.Scheduler$DisposeTask.run(Scheduler.java:578) 
at io.reactivex.internal.schedulers.ScheduledRunnable.run(ScheduledRunnable.java:66) 
at io.reactivex.internal.schedulers.ScheduledRunnable.call(ScheduledRunnable.java:57) 
at java.util.concurrent.FutureTask.run(FutureTask.java:266) 
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:301) 
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 
at java.lang.Thread.run(Thread.java:923) 
Caused by: com.bumptech.glide.load.engine.GlideException: Failed to load resource
```