Do not end the whole androidqf run on an encrypted backup.ab (#891)

from_ab() raises InvalidAndroidBackup instead of exiting when it runs as a
sub-command, which check-androidqf catches to skip the backup modules. The
two password branches still called sys.exit(1) unconditionally, and since
run_backup_cmd() runs inside finish(), that ended the parent run before the
intrusion-logs command and before the timeline, alerts, urls, info and run
manifest were stored — leaving an output directory that looks complete but
has no alerts.json.

Also drop "as backup.ab is malformed" from the skip warning: it covers a
missing or wrong password too.

tests/android/test_check_backup_optional_failure.py

@@ -0,0 +1,39 @@
+# Mobile Verification Toolkit (MVT)
+# Copyright (c) 2021-2023 The MVT Authors.
+# Use of this software is governed by the MVT License 1.1 that can be found at
+#   https://license.mvt.re/1.1/
+"""An encrypted backup.ab must not take the whole check-androidqf run with it.
+
+`CmdAndroidCheckBackup.from_ab()` already raises `InvalidAndroidBackup` instead
+of exiting when it runs as a sub-command (`check-androidqf` catches that and
+skips the backup modules), for a wrong file format and for a parse error. The
+password branches used to call `sys.exit(1)` unconditionally, which ends the
+parent run inside `finish()` — before the intrusion-logs command and before the
+timeline, alerts, urls, info and run-manifest are written.
+"""
+
+import pytest
+
+from mvt.android.cmd_check_backup import CmdAndroidCheckBackup, InvalidAndroidBackup
+
+ENCRYPTED_AB_HEADER = b"ANDROID BACKUP\n5\n0\nAES-256\n" + b"\x00" * 64
+
+
+class TestCheckBackupOptionalFailure:
+    def _cmd(self, tmp_path, sub_command):
+        return CmdAndroidCheckBackup(
+            target_path=None,
+            results_path=str(tmp_path),
+            module_options={"interactive": False},
+            sub_command=sub_command,
+        )
+
+    def test_missing_password_raises_when_nested(self, tmp_path):
+        cmd = self._cmd(tmp_path, sub_command=True)
+        with pytest.raises(InvalidAndroidBackup):
+            cmd.from_ab(ENCRYPTED_AB_HEADER)
+
+    def test_missing_password_still_exits_on_its_own_command(self, tmp_path):
+        cmd = self._cmd(tmp_path, sub_command=False)
+        with pytest.raises(SystemExit):
+            cmd.from_ab(ENCRYPTED_AB_HEADER)