mute print statements when using --dag to avoid "Error: <stdin>: syntax error in line 1 near 'Write'"

[Blosberg]

Normally the pipeline outputs warnings to screen if write permission is denied in the refgenome folder; but when outputting a DAG, this first line breaks the format for the vector file.

Error looks like this:
Error: <stdin>: syntax error in line 1 near 'Write'
print statements should be muted under these conditions (or maybe only output error message when writing is actually needed but can't be performed).