Ambiguous characters

[mjsull]

When downloading Escherichia from enterobase an ambiguous base is reported but download_enterobase runs to completion.
When you try to call variants using that database you get a dictionary key error (and the program crashes).
This is fixed by removing the offending Loci entry from the downloaded FASTAs and rebuilding with build_db.
It might make more sense either not to complete the creation of the database, or skip that locus in "call" as well.