Mapping fails when a target does not appear in any variants. #679
Description
When a score set (at least a reference-based one) has targets that do not appear in the variants, the mapping job fails at line 127 of /src/api/routers/map.py, because the key target_gene is not found in records[target_gene].
Ensuring that a score set has no target that does not actually appear in its variants may be reasonable, but if we decide to do this, the rule should be applied when variants are validated. The mapper client should handle this circumstance gracefully.