Make "arrayjoin: not one band or 4 bands" error report more helpful

Is your feature request related to a problem? Please describe. When getting this error on a script someone else wrote it’s not obvious what’s happening and what the problem is without searching an explanation on the web.

Describe the solution you’d like Adding something like

You may need to convert all images to the same color space. pyvips.Image.colourspace function may help with that.

Additional context Trying to just generate a text on a transparent background is unnecessarily complex and lacks an example that’s easy to find.

Code fix example

                vips_image = Vips.Image.pngload(png_pathname)
                if vips_image.interpretation != 'srgb':
                    vips_image = vips_image.colourspace('srgb')

[jcupitt]

Args should link to enums now, eg.:

https://libvips.github.io/pyvips/vimage.html#pyvips.Image.colourspace

I credited you in the changelog.

[jcupitt]

I made an enhancement issue for linking enum args to the relevant enum, thanks for the suggestion.