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 
[optional]
Not sure if we should actually localize exception messages.
If not it would mean we need to call the t() method in the JS side and trick the translation bot into translating these anyway. (which is not that elegant)

@wakeup can you help to clarify ?