Raise error when passing in wrong files

There are 2 issues that have the same error

https://github.com/biocore/mmvec/issues/103
https://github.com/knightlab-analyses/multiomic-cooccurrences/issues/13

This issue happens because there are no iterations that happen, either due to misspecified batch size or wrong input files. We'll need to have an error message raised for this.

Start by reproducing the no-iteration cases described in issues #103 and #13, including misspecified batch sizes and wrong input files. Trace the execution path that completes without iterations and identify where the error should be raised. Done means invalid inputs or configurations no longer finish silently and instead produce an actionable error message.

Written by the indexing model from the issue text.