I just noticed that if I dont enter any value for the executor field when
creating a slave and press SAVE i get the below ugly text:
Status Code: 500
Exception:
Stacktrace:

java.lang.IllegalArgumentException: hudson.model.Descriptor$FormException:
Invalid slave configuration for local. Invalid # of executors.

Couldnt the executor field's checkurl() method also work with empty/null
strings? This would mean that the warning would show when starting to add a new
slave, but I dont think that is a problem. The warning is better than a 500
page. Just let me know if you agree, and then I can fix it.