BehaviorSpace error message and documentation improvements

[ERSUCC]

Fixed a confusing error message in the BehaviorSpace GUI dialog, also made some clarifications and improvements to the BehaviorSpace documentation.

[brandesNW]

please fix the conflicts in the property file.
One way to find them is
git diff hexy netlogo-gui/resources/i18n/GUI_Strings_en.properties
The difference between "{0}" and {0} is what stands out. I think the latter is correct. can you fix and test? There are some other messages that use "{0}". If they are in error they should be fixed with a separate PR.