gnomad_vcf_to_protein warnings for intron variants

`variation/gnomad_vcf_to_protein?q=7-55142262-G-A` returns `Unable to get transcripts given NC_000007.14 and 55142262` in its warnings. This is because it maps to a location on an intron rather than an exon. It'd be more useful to have the warnings contain a message saying that it's unable to translate to protein variation since it's an intron variant

Start by calling the variation/gnomad_vcf_to_protein endpoint with q=7-55142262-G-A and trace where its warnings are assembled. Update the warning for an intron mapping to state that the variant cannot be translated to a protein variation, then verify that this input produces the clearer message.

Written by the indexing model from the issue text.