Bugfix JS: Catch error from GetLegendGraphics

[rldhont]

GetLegendGraphics can return response with code 400 and xml content like this

<?xml version="1.0" encoding="UTF-8"?>
<ServiceExceptionReport version="1.3.0" xmlns="http://www.opengis.net/ogc">
 <ServiceException code="LayerNotDefined">The layer ' routes et chemins' does not exist.</ServiceException>
</ServiceExceptionReport>

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