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/test/java/de/jeanlucmakiola/clockula/ui/settings/BackupViewModelTest.kt

@@ -3,6 +3,7 @@ package de.jeanlucmakiola.clockula.ui.settings
 import com.google.common.truth.Truth.assertThat
 import de.jeanlucmakiola.clockula.backup.BackupFiles
 import de.jeanlucmakiola.clockula.data.backup.BackupCodec
+import de.jeanlucmakiola.clockula.data.backup.BackupProblem
 import de.jeanlucmakiola.clockula.testing.MainDispatcherExtension
 import de.jeanlucmakiola.clockula.testing.backupHarness
 import kotlinx.coroutines.flow.first
@@ -31,7 +32,7 @@ class BackupViewModelTest {
 
         viewModel.onExportFolderPicked("content://folder")
 
-        assertThat(viewModel.events.first()).isEqualTo(BackupEvent.ExportFailed("provider unavailable"))
+        assertThat(viewModel.events.first()).isEqualTo(BackupEvent.ExportFailed)
         assertThat(viewModel.busy.value).isFalse()
     }
 
@@ -49,7 +50,7 @@ class BackupViewModelTest {
 
         viewModel.onImportFilePicked("content://backup")
 
-        assertThat(viewModel.events.first()).isEqualTo(BackupEvent.ImportFailed("database unavailable"))
+        assertThat(viewModel.events.first()).isEqualTo(BackupEvent.ImportFailed(BackupProblem.NOT_APPLIED))
         assertThat(viewModel.busy.value).isFalse()
     }
 }