Improve error message when request too large

[dopplershift]

The error:

HTTPError: Error accessing https://thredds.ucar.edu/thredds/ncss/grib/NCEP/GFS/Global_0p5deg/Best?var=Pressure_reduced_to_MSL_msl&var=Geopotential_height_isobaric&time_start=2021-06-17T12%3A00%3A00&time_end=2021-06-21T18%3A00%3A00&addLatLon=True&accept=netcdf
Server Error (400: NCSS response too large = 1528329600 max = 629145600)

seems to be somewhat confusing to our users. It might be helpful to detect that error and suggest that they restrict their request to reduce its size.