nfs-server.service fails to start due to dependency issues or bad unit file configuration

Issue

• nfs-server.service fails to start with messages similar or related to:

  Failed to start nfs-server.service: Unit is not loaded properly: Bad message.
  
  or
  
  Failed to start nfs-server.service: Unit proc-fs-nfsd.mount has a bad unit file setting.
  
  or 
  
  A dependency job for nfs-server.service failed. See 'journalctl -xe' for details.
  

• Checking the status of proc-fs-nfsd.mount or system logs show the following errors:

  systemd[1]: [/etc/systemd/system/proc-fs-nfsd.mount:1] Assignment outside of section. Ignoring.
  systemd[1]: proc-fs-nfsd.mount's What setting is missing. Refusing.
  
  or
  
  systemd[1]: /etc/systemd/system/proc-fs-nfsd.mount:1: Assignment outside of section. Ignoring.
  systemd[1]: proc-fs-nfsd.mount: What= setting is missing. Refusing.