"Empty" batch validator report files should contain a warning message

[leicray]

Describe the bug
By default, a batch validator report file contains the job ID on the first line and the tab-delimited column headers on the next line. Subsequent lines contain the actual tab-delimited results corresponding to the column headers.

In certain circumstances, the batch validator may return no actual results but no warning to the user is included to say why that might be.

To Reproduce
Steps to reproduce the behaviour:

1. Open the batch validator.
2. Enter the variant description NM_000088.4:c.589G>T into the "Input Variant Descriptions" input box
3. Enter the transcript ID NM_000089.4 into the "Optional - limit to specific transcripts (see our Genes to Transcripts tool). or return select transcripts only" input box.
4. Enter your email address, select GRCh38, and select refseq.
5. Click the Submit button and wait for the confirmation emails for submission of the job and the return of the results.
6. Open the results email, download the results file and open it. The file will comprise only the submission ID and the column headers.

Optionally, the same type of error can be generated by entering the gene symbol COL1A2 into "Limit search, optionally, to specific genes (use HGNC gene symbols)" input box.

The lack of results for the fully described example is because the variant description NM_000088.4:c.589G>T corresponds to a variant in the COL1A1 gene, but NM_000089.4 is a transcript for the COL1A2 gene. The second example rests on the same lack of correspondence between the query description and the gene defined in the limit.

Expected behaviour
A warning message to the user ought to be included in batch validator results files when no results are found that meet the submitted criteria.