Handle storages not being available in webui and webdav

Adds `\OCP\Files\StorageNotAvailableException` which can be thrown by storage backends for when previously configured storage backends are (temporary) not available.

This exception is handled by both the web ui (show an error message) and WebDAV (throw a 503)

For #9088 

cc @PVince81 @DeepDiver1975 @schiesbn 
Yes, I produced the exception. I think we should leave it like this. This is also not written into log, so I see no problem here.