add check for input taxa that is not present within ALA database

[due23]

galah.atlas_counts("Ailuropoda melanoleuca") returns KeyError: 'taxonConceptID'

R equivalent behaves in the following way:

Also the same when multiple taxa are used and one is not in the ALA database: galah.atlas_counts(["Dasyurus hallucatus", "Ailuropoda melanoleuca", "Centrostephanus rodgersii"], separate=True) -> instead should behave as below (ie. ignore the taxon not present and return the counts for the other present species):

[acbuyan]

added checks to see if the taxa exists in the atlas