Errors

Every failure is a CloudStorageException carrying one CloudError: NotAvailable, NeedsConsent, Offline, StorageFull, AuthRevoked, NotFound, Transport. SyncEngine.sync never throws for cloud failures; it returns SyncOutcome.Failed(error).

Suggested user copy

Every adopter ends up writing these strings. A starting point that Vocabloot ships:

CloudError	Shown when	Suggested line
NotAvailable	no iCloud account, or Play Services missing	"Sign in to iCloud on this iPhone to back up." / "Google Play services is needed for backup."
NeedsConsent	Drive grant missing or revoked	"Allow Vocabloot to use your Google Drive to keep a backup."
Offline	no network	"Offline. Will retry when you are back online."
StorageFull	iCloud or Drive quota reached	"Your iCloud storage is full. Free some space to keep backing up."
AuthRevoked	token rejected	"Backup lost access to your account. Turn it off and on again."
NotFound	a file vanished between list and read	(retry silently; the next sync heals it)
Transport	anything else	"Backup hit a problem. Will retry."

SyncOutcome.Unavailable(reason) carries NoAccount, NeedsConsent, RestorePending, WriteHeld or ShrinkSuspected. RestorePending means the device is empty and the cloud is not, so offer a restore instead of uploading. ShrinkSuspected means this device would delete most of the backup: "This phone has fewer words than your backup. Backup is paused until you choose." with two actions, restore the backup or replace it with this phone's words (retry with allowShrink = true).