Warn the user in the barplot legends about tips that have no feature metadata / are not in the BIOM table

[fedarko]

A warning is shown for continuous barplot legends when some tips have non-numeric feature metadata for the field:

(this is with a test version of the MP dataset where I replaced a few tips' Confidence values with non-numeric junk.)

It would be good to also show a warning about tips that do not contain any feature metadata (for which no bars are drawn in a feature metadata barplot, at least assuming that some color / length encoding is used), as well as about tips that are not present in the BIOM table (for which no bars are drawn in a sample metadata barplot, since we can't associate the tip with any samples). For 16S datasets, both of these cases will probably only happen when --p-no-filter-unobserved-features-from-phylogeny is used.

The main challenge in making the code for this work will probably just be passing the information of "there were tips without [feature|sample] metadata" to the barplot layer objects.