savecube: Giving false file extension for netcdf backend gives confusing axes in QGIS

When I am trying to save a cube with the following function call where I am mistyping the file extension I get a netcdf file, that has the wrong x and y axes values, when I open them in QGIS and I can't open the file again in YAXArrays, because it tries to load it as a zarr file.
I think it would be safer to throw an error, when the extension is wrong than to write data, that is either unreadable or gives the wrong results.
Or we could add the right file extension and accept the filepath without the file extension.

Start by reproducing the savecube case with a mistyped extension for the netcdf backend and inspect how the filepath extension selects the storage backend. Done should mean the invalid extension no longer produces misleading axes or a file YAXArrays cannot reopen, either by reporting an error or by consistently adding the appropriate extension.