Improve documentation of lastgenre.prefer_specific's dependency on lastgenre.canonical?

[zsinskri]

Problem

When lastgenre.prefer_specific is enabled but lastgenre.canonical is not beet lastgenre will fail to apply any genres. This could be better documented.

Error message

It would be nice if there would be a error message when prefer_specific is enabled but canonical is not.
The current behaviour:

$ # with prefer_specific but without canonical:
$ beet lastgenre LITE Cubic
lastgenre: genre for album LITE - Cubic (None):
$ # compare: with canonical:
$ beet lastgenre LITE Cubic
lastgenre: genre for album LITE - Cubic (artist): Post-Rock

Suggested behaviour:

$ # with prefer_specific but without canonical:
$ beet lastgenre LITE Cubic
lastgenre: ERROR: `lastgenre.prefer_specific` is enabled but `lastgenre.canonical` is not. Please either disable `prefer_specific` or enable `canonical`!

Documentation

Currently the documentation only hints at this requirement. One could possibly make this clearer.

Alternatives

Alternatively the default canonicalisation tree could just be loaded. Care must be taken to only use it for sorting, not canonicalisation.