Improve communication between purity and assign

[cmorganl]

treesapp purity calls treesapp assign to classify the reference sequences. The warning and error messages are verbose in each, but if there is an error in treesapp assign the exact cause of the error, and whether it truly is a bug or other issue can get muddled.

For example, when there are no alignments found in the reference database, a warning is correctly issued followed by three errors:

WARNING:
No alignments found! TreeSAPP is exiting now.
ERROR - commands, line 1176:
TreeSAPP failed.
ERROR - commands, line 1188:
marker_contig_map.tsv is missing from output directory 'intermediates/assign/final_outputs'
Please remove this directory and re-run.

treesapp assign needs to return an error code that can be properly interpreted by the wrapping subcommand.