amass enum -nf flag causes amass to abort -- suggest log error & continue

[prisoner881]

Using the -nf flag, you can feed a list of FQDN's to amass that you discovered via other means. However, if the flag points to a non-existent or empty file, amass aborts the enumeration.

I suggest a slightly different approach. Instead of aborting, amass should log the missing/empty file as an error in AMASS.LOG and continue the enumeration. There's no reason to abort the enumeration since this file is optional.