Give proper error message for `get_num_lanes`

[genericdata]

GENEFLOW/main.nf

Lines 11 to 27 in 4889588

	def cmdList = [
	"python3",
	"-c",
	"from slime import get_num_lanes; print(get_num_lanes('${fcid}'))"
	]
	def proc = cmdList.execute(envVars, null)
	proc.waitFor()
	if (proc.exitValue() != 0) {
	error "get_num_lanes failed: ${proc.err.text}"
	}
	def lines = proc.in.text.readLines()
	if (!lines) {
	error "No output from get_num_lanes()"
	}
	def num_lanes = lines.last().trim().toInteger()
	def lanes = channel.from(1..num_lanes)

When there are two folders matching */*${fcid} or no lanes are found the error is not clear.