SecureBackup: improve error flow when backup state cannot be retrieved, and add tests.

libraries/matrix/test/src/main/kotlin/io/element/android/libraries/matrix/test/encryption/FakeEncryptionService.kt

@@ -34,6 +34,7 @@ class FakeEncryptionService : EncryptionService {
     private var waitForBackupUploadSteadyStateFlow: Flow<BackupUploadState> = flowOf()
 
     private var fixRecoveryIssuesFailure: Exception? = null
+    private var doesBackupExistOnServerResult: Result<Boolean> = Result.success(true)
 
     override suspend fun enableBackups(): Result<Unit> = simulateLongTask {
         return Result.success(Unit)
@@ -52,8 +53,12 @@ class FakeEncryptionService : EncryptionService {
         return Result.success(Unit)
     }
 
+    fun givenDoesBackupExistOnServerResult(result: Result<Boolean>) {
+        doesBackupExistOnServerResult = result
+    }
+
     override suspend fun doesBackupExistOnServer(): Result<Boolean> = simulateLongTask {
-        return Result.success(true)
+        return doesBackupExistOnServerResult
     }
 
     override suspend fun fixRecoveryIssues(recoveryKey: String): Result<Unit> = simulateLongTask {