Improve error reporting for not_done jobs in buildah

If a job times out in buildah, very little information is reported.
This patch adds a list of exceptions that were thrown by incomplete
jobs to the SystemError exception that gets raised.

Change-Id: I3526144934fd04a9996975f6e8610c2029d16f67

tripleo_common/image/builder/buildah.py

@@ -186,11 +186,21 @@ class BuildahBuilder(base.BaseBuilder):
                     raise SystemError(job._exception)
             else:
                 if not_done:
-                    raise SystemError(
+                    error_msg = ('The following jobs were '
-                        'The following jobs were incomplete: {}'.format(
+                                 'incomplete: {}'.format(not_done))
-                            not_done
+
-                        )
+                    exceptions_raised = [job._exception for job in not_done
-                    )
+                                         if job._exception]
+
+                    if exceptions_raised:
+                        error_msg = error_msg + os.linesep + (
+                            "%(raised_count)d of the incomplete "
+                            "jobs threw exceptions: %(exceptions)s" %
+                            {'raised_count': len(exceptions_raised),
+                             'exceptions': exceptions_raised})
+
+                    raise SystemError(error_msg)
+
         elif isinstance(deps, (dict,)):
             for container in deps:
                 self._generate_container(container)

tripleo_common/tests/image/builder/test_buildah.py

@@ -38,7 +38,7 @@ BUILD_ALL_STR_CONTAINER = 'container1'
 
 
 class ThreadPoolExecutorReturn(object):
-    pass
+    _exception = None
 
 
 class ThreadPoolExecutorReturnFailed(object):