MacOS Sierra - localhost not found while using Hugo (not related to Apache)
The key to this error was the following:


  127.0.0.1:1313 works, but then all of the links fail as they try to resolve back to localhost.


That gave the hint - accessing via IP works but not via hostname.  Thankfully, the /etc/hosts file was also included for examination.  In it we see the following line (1st line):

127.0.0.1       localhostw


The trailing "w" at the end of the hostname localhost will cause the error.  In effect, localhost wasn't defined, but localhostw was.

Removing the "w" will solve the issue.