Add input validation for _from_gnomad

[korikuzma]

I made a similar issue for translator validation in our hackathon repo.

7-7-helloworld-T is not a valid gnomAD VCF, but we're still able to translate to a VRS Allele as seen below:

We should add some additional validation checks for the input

[korikuzma]

@ahwagner In the notebooks, I saw this description:

I'm now wondering if we should provide a parameter on whether or not to do validation rather than always requiring valid input.

[ahwagner]

I think we should check and throw a warning. If somebody provides a reference nucleotide for translation, and that reference does not match the sequence and location provided, it is an indication something may be wrong in their workflow. I think it is okay to parameterize this check if people want to suppress that behavior, but I think it should be active by default.