Add config-error reporter and report config errors to DB

This adds a config-error pipeline reporter configuration option and
now also reports config errors and merge conflicts to the database
as buildset failures.

The driving use case is that if periodic pipelines encounter config
errors (such as being unable to freeze a job graph), they might send
email if configured to send email on merge conflicts, but otherwise
their results are not reported to the database.

To make this more visible, first we need Zuul pipelines to report
buildset ends to the database in more cases -- currently we typically
only report a buildset end if there are jobs (and so a buildset start),
or in some other special cases.  This change adds config errors and
merge conflicts to the set of cases where we report a buildset end.

Because of some shortcuts previously taken, that would end up reporting
a merge conflict message to the database instead of the actual error
message.  To resolve this, we add a new config-error reporter action
and adjust the config error reporter handling path to use it instead
of the merge-conflicts action.

Tests of this as well as the merge-conflicts code path are added.

Finally, a small debug aid is added to the GerritReporter so that we
can easily see in the logs which reporter action was used.

Change-Id: I805c26a88675bf15ae9d0d6c8999b178185e4f1f

1 files changed, 6 insertions, 0 deletions

diff --git a/releasenotes/notes/config-error-reporter-34887223d91544d1.yaml b/releasenotes/notes/config-error-reporter-34887223d91544d1.yaml
new file mode 100644
index 000000000..51690f2fa
--- /dev/null
+++ b/releasenotes/notes/config-error-reporter-34887223d91544d1.yaml
@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    A new :attr:`pipeline.config-error` pipeline reporter is available
+    for customizing reporter actions related to Zuul configuration
+    errors.