core: fix error log for run(steps)

Before:
  - debug: run killed with: {}
After:
  - debug: run killed with: TypeError: Cannot read properties of undefined (reading 'condition')

Change-Id: I640cb5dc0070a66537bd0188cb108be1ab2f7e12
Signed-off-by: Alexander Martinz <amartinz@shiftphones.com>

src/core/core.js

@@ -315,7 +315,7 @@ class Core {
       .reduce((chain, next) => chain.then(next), Promise.resolve())
       .catch(error => {
         // used for killing the run, no actual errors should be escalated here
-        log.debug(`run killed with: ${JSON.stringify(error)}`);
+        log.debug("run killed with: " + error);
         log.warn("aborting run...");
       });
   }