Use download format constant.

Removed unused variable.
DownloadController no longer uses Layer to retrieve information.
BodaacDownloadHandler no longer passes-on layerId
BodaacDownloadHandler no longer passes-on serverUrl
Moving the usage of downloadFilename from ProxiedRequest to RequestProxyingController.

This was causing problems because there were values in params (namely 'downloadFilename') which were being included in the external URL but which were only intended for the Portal to use. Geoserver was balking when it got query string parameters it doesn't know about.
Handle exceptions in the external request when trying to download source NetCDF files.

Also removed paramater values from 'params' as we used them so they weren't also forwarded on with the external URL.