BF: String formatting in exception message (#8206)
BF: String formatting in exception message (#8206)

#8194 introduced changes to `mne.viz.backends._utils.VALID_3D_BACKENDS`.

Now, if `viz.backends.renderer._get_3d_backend()`
cannot find a valid backend, the string formatting
doesn't work (`VALID_3D_BACKENDS` is a tuple, and we're trying to format
the string with only a single `%s`.

This commit not only fixes that, but improves formatting ever so
slightly.
wmvanvliet/mne-python,Teekuningas/mne-python,pravsripad/mne-python,bloyl/mne-python,larsoner/mne-python,Teekuningas/mne-python,drammock/mne-python,rkmaddox/mne-python,wmvanvliet/mne-python,mne-tools/mne-python,mne-tools/mne-python,mne-tools/mne-python,kingjr/mne-python,larsoner/mne-python,cjayb/mne-python,kambysese/mne-python,Eric89GXL/mne-python,olafhauk/mne-python,olafhauk/mne-python,pravsripad/mne-python,cjayb/mne-python,olafhauk/mne-python,rkmaddox/mne-python,pravsripad/mne-python,wmvanvliet/mne-python,Teekuningas/mne-python,drammock/mne-python,drammock/mne-python,Eric89GXL/mne-python,bloyl/mne-python,larsoner/mne-python,kingjr/mne-python,kingjr/mne-python,kambysese/mne-python