ENH: Improve error message for vak.split.algorithms.brute_force

[NickleDave]

@lmpascual ran into an issue where this error message got thrown by vak.split.algorithms.brute_force

vak/src/vak/split/algorithms/bruteforce.py

Line 255 in 4199bc5

"Could not find subsets of sufficient duration in "

For a user who doesn't have some of the context of what prep is doing under the hood here, this is kind of confusing.

It might sound like it's telling them they need to have 5000 renditions of a certain syllable, for instance.

The explanation should be a bit clearer, with more context. Something like, "The algorithm that creates splits of the dataset for training, validation, etc., did not converge in the maximum number of iterations". It should also include information to help the user fix the problem, such as: "Check that your labelset does not include vary rare labels, which may make it hard to find splits that all contain at least one instance of each label. You might also consider removing any segments labeled as a 'background' or 'trash' category if you want the trained model to ignore those classes; if the segments are removed they will be given the default 'background' label instead, as long as you have unlabeled segments in your dataset such as silent gaps between vocalizations."