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

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.

Start by tracing the existing continuous barplot legend warning and the barplot layer objects. Follow where missing feature metadata and missing BIOM-table tips are detected, then determine how that information reaches the legends. Done means the relevant barplot legends warn users about each missing-data case without changing the existing non-numeric metadata warning.

Written by the indexing model from the issue text.