improvement for error/warning message for wrong delimiter

[anoronh4]

i had the following error:

nextflow.validation.SchemaValidationException: The following errors have been detected in /home/noronhaa/pytest_workflow_wrrzqigf/run_test_fillouts/tests/test_fillout/data/input.tsv:

-> Entry 1: Missing required field(s): maf, sample

which seemed weird to me because my input sheet was the following:

sample,maf
SAMPLE_PAIRED_END,tests/test_fillout/data/dna.maf

turns out it's because it was because it was a comma-delimited file, but nf-schema is expecting tab delimiters due to the file extension. it might be helpful to users if the error were more informative about the delimiters. in this case the error might have also said something like "no tabs found, expecting tabs, extension is .tsv".