mgmtd: improve error logging when processing config files

[mjstapp]

It looks like mgmtd silently ignores some config lines if they appear invalid. It would be better to get explicit error output when a config line is invalid, can't be parsed, etc.

An example would be an invalid IP address in an interface configuration statement, like this (note ":1" instead of "::1")

interface eth0
  ipv6 address 1111:2222:1/64