feature request: change raised error message when dealing with wrongly named .txt files

[toobiwankenobi]

I had a typo in one of my acquisition .txt files names (iMC data) which led to the following error message:

Error in .local(x, y, ...) : Invalid barcode channel(s) specified.

Reference to the corresponding code:

CATALYST/R/assignPrelim.R

Lines 49 to 50 in 5c9c2df

	if (any(!(bc_ms %in% ms)))
	stop("Invalid barcode channel(s) specified.")

It took me quite some time (actually @votti found the causing error) on how to fix the issue. I think it would be more intuitive and therefore user-friendly to raise a more meaningful error (e.g. invalid metal name, check file names). I don't know if this also relates to sMC data but if yes, I think it would make sense to change the mentioned error message.

Thank you very much for your work!