NPE: general enhancements

From google play console:
```
Caused by: java.lang.NullPointerException:
at com.owncloud.android.ui.activity.FileDisplayActivity$DownloadFinishReceiver.onReceive (FileDisplayActivity.java:1432)
```
which is
` getStorageManager().getFileByPath(mWaitingToSend.getRemotePath())`

This method can return null if the file does not exist in our database.

We could
- add @Nullable to method declaration
- throw IllegalArgumentException, which does not need to be handled
- throw FileNotFoundException, which needs to be handled

This is just an example, there are a lot more functions where we might return null and then have to check with "returnedObject != null".

This is not something we need to address now and should be discussed on case by case.