ZoomTo - need projection validation

[adube]

issue

The ZoomTo widget currently doesn't validate if the projection file exists when including it, which can result in throwing javascript errors or simply make the reprojection not working.

solution

Since it's not possible to detect if a file exists in xslt, do the validation in javascript by validating that the projection exists in the !Proj4js.defs object. If it doesn't, alert an error message.

widget status I consider this a required for the widget to be considered stable in 1.0. If not fixed, then mark as unstable and push ticket to 1.2.