When the area selected for export is too large to be downloaded from

the API then disable the export button and display a message rather
than forcibly disabling that export option. Closes #1996.

config/locales/en.yml

@@ -362,6 +362,9 @@ en:
       embeddable_html: "Embeddable HTML"
       licence: "Licence"
       export_details: 'OpenStreetMap data is licensed under the <a href="http://creativecommons.org/licenses/by-sa/2.0/">Creative Commons Attribution-ShareAlike 2.0 license</a>.'
+      too_large:
+        heading: "Area Too Large"
+        body: "This area is too large to be exported as OpenStreetMap XML Data. Please zoom in or select a smaller area."
       options: "Options"
       format: "Format"
       scale: "Scale"