fix: say why a restore failed in words that can be translated

The snackbar appended the codec's or the exception's own English text,
class names and all. Failures now carry a category (not a backup, from a
newer version, invalid data, too large, unreadable) that the screen maps
to a string resource; the detail stays in the result for tests.

app/src/main/res/values/strings.xml

@@ -329,6 +329,11 @@
     <string name="settings_backup_export_failed">Couldn\'t save the backup</string>
     <string name="settings_backup_import_succeeded">Backup restored</string>
     <string name="settings_backup_import_failed">Couldn\'t restore the backup</string>
+    <string name="settings_backup_import_failed_not_backup">Couldn\'t restore: this isn\'t a Clockula backup</string>
+    <string name="settings_backup_import_failed_newer">Couldn\'t restore: this backup is from a newer Clockula. Update the app first</string>
+    <string name="settings_backup_import_failed_invalid">Couldn\'t restore: the backup has invalid data in it</string>
+    <string name="settings_backup_import_failed_too_large">Couldn\'t restore: the backup is too large</string>
+    <string name="settings_backup_import_failed_unreadable">Couldn\'t restore: the file couldn\'t be opened</string>
     <string name="settings_backup_import_confirm_title">Replace everything?</string>
     <string name="settings_backup_import_confirm_message">This replaces all your current alarms, timers, world clocks and settings with what\'s in the backup file. This can\'t be undone.</string>
     <string name="settings_backup_import_confirm_action">Replace</string>