Handle missing explanation template.

Original description

explain/4 should succeed even if an required explanation template is missing.

Discussion

This is hard, if not impossible to realize. Let's suppose there are two possible traces for irrelevant(F): a(F) and b(F). But there is only an explanation template for b. Then the result would be:

1. an explanation using a and
2. an empty explanation.

Or, if only for a an explanation template is known, we have an empty explanation followed by an explanation using b. Either way, we have two explanations, one of which is empty. This is no sensible behavior.

Desired behavior

Have a graceful API reply if a requested item is irrelevant but explain/4 cannot produce an explanation. The reply should be (following the current naming convention):

{
  "explanations": [],
  "further_explanations": false
}