Completion closure on putData to upload a file to Firebase Storage is never called

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### [REQUIRED] Step 1: Describe your environment

* Xcode version: 11.0
* Firebase SDK version: Latest
* Firebase Component: Storage
* Component version: Latest
* Installation method: CocoaPods

### [REQUIRED] Step 2: Describe the problem

When running a `putData` method in an iPhone without any internet connection it never calls the completion callback.

When monitoring the progress/status of an upload task it never gets called too, i.e. we as developers will get stuck and won't get any response from the Firebase SDK when trying to upload a file without internet connection. We can't just show a connection error for our users because we don't get notified when it does happen, and the "completion" that should be called even when there's an error doesn't get called.

#### Steps to reproduce:

Just try to upload a file without any internet connection.

#### Relevant Code:

No need.