Raise error when writing an RGBA colormap to a paletted TIFF

[sgillies]

Instead of failing silently. See #394.

[wmaiouiru]

So is this documentation not valid any more? https://github.com/mapbox/rasterio/blob/master/docs/colormaps.rst
Is there no way to create geotiff with rgba color map?

[sgillies]

@wmaiouiru the GeoTIFF format doesn't support RGBA colormaps and all alpha values given become 255. Would it help you if Rasterio accepted colormap tuples like (r, g, b, 255) and only raised a ValueError if you passed (r, g, b, a) where a was not 255?

[wmaiouiru]

@sgillies Thank you for the response. Even though it is not supported, I wonder if it is possible to supply a flag that acknowledge that you are creating something that is not quite supported and not raise a flag? But accepting (r,g,b,255) works too.