backend: don't eat the "build detail collecting" traceback

Use "log.exception()" which passes the traceback down to backend.log.
This helps me to debug some release-blocker issue right now, normally we
(when infra is OK) we shouldn't ever catch an exception there.

backend/copr_backend/background_worker_build.py

@@ -678,9 +678,9 @@ def _get_build_details(self, job):
                 }
             self.log.info("build details: %s", build_details)
         except Exception as e:
-            raise BackendError(
-                "Error while collecting built packages for {}: {}"
-                .format(job.task_id, str(e)))
+            self.log.exception("Can't collect build results for %s",
+                               job.task_id)
+            raise BackendError("Can not deduct build details") from e
 
         return build_details