also print traceback

IPython/testing/iptestcontroller.py

@@ -675,6 +675,8 @@ def run_iptestall(options):
                 cov.xml_report(outfile='ipy_coverage.xml')
             except CoverageException as e:
                 print('Generating coverage report failed. Are you running javascript tests only?')
+                import traceback
+                traceback.print_exc()
 
     if failed:
         # Ensure that our exit code indicates failure