Allow for custom candidate viability considerations

[zbraniecki]

Current JS based L10nRegistry rejects any candidate that has any file missing. This is a blunt strategy and it results in user perceivable experience degradation in cases where the user is loading a UI that has a lot of sources and one is missing (because it's new).

In such case L10nRegistry will reject such bundle and potentially flip to the next language which means that 1/13 of files missing result in 100% rejection of the locale.

One avenue to improve that is to fine-tune our problem solver to be more subtle about deciding when to reject a candidate. For example, the solver could consider a candidate viable if the number of resources is larger than 3, and the number of missing resources is lower than 2.

This would allow for a single missing file in L10nRegistry not to reject the whole candidate.

Alternatively, we could somehow mark which resources in a bundle are "critical" and bail when one of them is missing, while accepting missing non-critical resources. Or reverse - mark resources as "optional".
But that approach seems more manual and requires more maintenance over time.

My hope is that some simple heuristic will give us a good experience.