Use checkIfExists

[ewels]

The new checkIfExists option was added in nextflow-io/nextflow#666 and is now released. Would be great to use it!

See https://gitter.im/nextflow-io/nextflow?at=5ae09da61130fe3d361684a1 for reason:

star_index = Channel
        .fromPath(params.star_index)
        .ifEmpty { exit 1, "STAR index not found: ${params.star_index}" }

However, as far as I can see, this does not work if a wrong file path is given since fromPath() doesn't check for file existence and therefore, the channel is not empty.