#1112: sort_by validates field format and reports one with no data

[morphqdd]

The docblock of sort_by says it raises Fbe::Error if the property is nil, already set, or not a valid field name, but the body only checked nil, already-set, and is_a?(String). The sibling as validates against /\A[_a-z][a-zA-Z0-9_]*\z/.

That alone does not catch a typo like sort_by('nubmer'): the name is a syntactically valid field, so nothing rejects it. The query still runs, but filter_map { _1[@sorting]&.first } drops every row since none of the matched facts carry a nubmer property, so the repository looks exhausted and restarts, and nothing above debug is logged.

Added the same format check as uses, and made the per-repository sorted-values lookup raise Fbe::Error naming the field when matched facts exist but none of them carry it, instead of silently looking like an exhausted queue.

Closes #1112