suggestion: bustools count should more gracefully handle if a transcript exists in transcripts.txt file but not in t2g file

[Yenaled]

Right now, it would infinite loop if the --umi-gene option is enabled (without --umi-gene, it would output a mtx file where an erroneous entry is given the value of 0). This also should be fixed in the kb_python unit test where there's a mismatch between transcripts.txt and transcripts_to_genes.txt.