BUG: Fix #1608

split_otu_table.py would produce a directory with only mapping files and
no OTU tables, this was happening because the axis in the filter
operation was `'observation'` instead of `'sample'`.
MAINT/BUG: Add exception check to split_otu_table
TST: Add test for exception in split OTU-table
BUG/MAINT: Add exception catching at script
MAINT: Change ValueError to OTUTableSplitError
Merge pull request #1609 from ElDeveloper/issue-1608

BUG: Fix #1608