Check and improve error messages

[hdbeukel]

In #4 we made sure that the workflow fails when motif mapping files are not found. The message doesn't mention the file path where the motif mapping files where expected though. We should include the path MotMapsFile in the error message.

Wait to implement this until #3 is merged to avoid another conflict 😄

[hdbeukel]

In fact we should check all error messages. For example I noticed this is wrong:

de_files = Channel.fromPath("${DE_genes_dir}/*.txt")
                  .ifEmpty { error "Cannot find any directory: ${Set_genes_dir}" }

The error message refers to Set_genes_dir instead of DE_genes_dir. Also it says the directory is not found while it could be the directory is actually there but the files inside it are missing (or have a wrong extension).