Looking for errors in dig output files must not fail

If no errors in dig output files of failed tests are found, TSAN file
parsing won't occur.

(cherry picked from commit 28a0df3081)

.gitlab-ci.yml

@@ -323,7 +323,7 @@ stages:
     - REALSOURCEDIR="$PWD"
     - test -n "${OUT_OF_TREE_WORKSPACE}" && cd "${OUT_OF_TREE_WORKSPACE}"
     - cat bin/tests/system/test-suite.log
-    - find bin/tests/system -name "*dig.*" | xargs grep "error"
+    - find bin/tests/system -name "*dig.*" | xargs grep "error" || true
     - *find_python
     - >
       "$PYTHON" "$REALSOURCEDIR"/bin/tests/convert-trs-to-junit.py . > "$CI_PROJECT_DIR"/junit.xml
@@ -346,7 +346,7 @@ stages:
   <<: *system_test_common
   after_script:
     - cat bin/tests/system/test-suite.log
-    - find bin/tests/system -name "*dig.*" | xargs grep "error"
+    - find bin/tests/system -name "*dig.*" | xargs grep "error" || true
     - *find_python
     - *parse_tsan
     - >