Improve error-handling in modules/samplesheet.nf when the filename doesn't match a regex

[ahmed-said-jax]

Line 38 in modules/samplesheet.nf assumes that the filename matches the provided regex, and then tries to index into the string. BUT the string doesn't exist, so you just get an InvocationTargetException that points at an IndexOutOfBoundsException, which basically tells you nothing. We can actually just CHECK if the regex matched, and if not, we can provide a helpful error, saying "change the filename" or something like that.

Maybe this behavior can be fundamentally improved - do we need to match a regex? Can we just assume that every file in the directory is actually a fastq, regardless of its name? I think the name of the file is somehow important to cellranger, so that might not be an option.