Provide migration guidance and pause transfers when SD card or storage path changes

### Actual behaviour
- change the SD Card (incl. Backup)
- all uploads / downloads fails without giving a hint about the cause

### Expected behaviour
- When Changing an SD Card and storage path gets outdated / inaccessible, guide the user by saying "Storage Folder is not accessible" to the setting to allow him to move over to a new path
- pause downloads / uploads unless a valid path is set to avoid error notifications

possibly:
- auto-detect a changed SD Card an just ask if we should temporarily ignore this - means pause nextcloud - or if we should notice the swap

PLUS:
- When storage path set is no longer available in selection for new users, give a hint to let user set a new (secure) path (incl. migration << issue https://github.com/nextcloud/android/issues/1048)

### Steps to reproduce
1. existing installation with external external SD Card set as storage path:
`/storage/0381-09EC/Android/data/com.nextcloud.client/files` (aside that this is too technical https://github.com/nextcloud/android/issues/1088) it is now outdated?! (referring to https://github.com/nextcloud/android/issues/1048) << no Kitakat+ SD Card formatted as internal Storage extension!
3. change the SD Card (possibly user story: change to use a bigger one)
4. uploads / downloads fail
5. you have to manually find the cause and setting yourself,

Actually it is even not possible for technical advised users to just select the new SD Card which has a 1:1 copy of the old
due to the fact, that there are now paths in the selection, that doesn't allow to reuse the copied data on the SD Card (referring to path mentioned in step 1)
** `/storage/emulated/0/Android/media/com.nextcloud.client` aka internal Storage
** `/storage/B840-C408/Android/media/com.nextcloud.client` aka external Storage / new SD Card
** `/data/user/0/com.nextcloud.client/files` ?!? whatever
** `/data/emulated/0/Android/data/com.nextcloud.client/files` (internal again, but path different to option 1 but would match what was set on my previous SD Card)

^ user: lost

### Environment data
Android version: 7.1.1

Device model: Moto G 4 Play

Stock or customized system: Stock

Nextcloud app version: 3.2.0 (actually latest)