No error message when disk is full

[AutoImport-spectas]

When there is almost disk free on the phone, DAVDroid stops syncing without notifying about the reason. A short message like “syncing failed: no space left on disk” would be great. It took me some time to figure this out.

Thanks for the app, it is great!
Spectas

[rfc2822]

Merged into #182, please follow up there.