more accurate getPrevalence() warning for deletions

[TKMarkCheng]

A response to #81 which raised the issue of the inaccurate and confusing error message in the GetGenomicsData() API:

In getGenomicData(query_url = "prevalence-by-location", pangolin_lineage = pangolin_lineage, :
Mutations should be specified in the format gene:mutation, like "S:E484K". The following mutations are not in the specified format: S:DEL69/70

1. Improved Grepl Regex which includes the scenario where a deletion mutation is queried.
2. Improved error message to show the preferred format for deletions. "{gene}:DEL{start codon}/{end codon}"