small improvement: more informative error handling for facetsSuite::ccf_annotate_maf

[ndfriedman]

I've noticed that facetsSuite::ccf_annotate_maf fails when in a couple of cases (purity=NA, no mutations in the maf) with an identical uninformative error:

Error in if (is.na(total_copies)) { : argument is of length zero Calls: <Anonymous> -> [ -> [.data.table -> expected_mutant_copies Execution halted

It would be helpful if instead we got a message about facets not being able to estimate a purity, having 0 mutations etc and that the program would either fail with an error or return an unannotated maf