Error message control

I would like to have a feature to allow me to control precisely what the error message is if something fails, suppressing the normal NextFlow error message and stack track.

Sometime the workflow runs into an error because of some problem with the data and I want to print a clear error message to the user to explain the problem and how to fix it. However, the normal error message is very verbose and for someone who is not the developer of the workflow very confusing. This is particularly the case when the script being run in a template where the whole templatised code gets dumped out as the "script".

What I'd want is a directive like

errorMessage 45 "pheno_err_msg.py $fname"

which would mean "if any step in the script returns errror code 45, call the script bin/pheno_err_msg.py"

Review the proposed errorMessage directive and the example bin/pheno_err_msg.py handler. Define how an exit code such as 45 selects the handler, how $fname is passed, and how the normal Nextflow error message and stack trace are suppressed. Done means a workflow can provide a clear custom message for the failing data case.