sysv-generator: better error reporting (#3977)

sysv-generator: better error reporting (#3977)

Currently in the journal you get messages without context like:
systemd-sysv-generator[$pid]: Failed to build name: Invalid argument

When parsing the init script, show the file and line number where the
error was found. At the same time, add more context information if
available.

Thus turning the message into something like:
systemd-sysv-generator[$pid]: [/etc/init.d/root-system-proofd:13] Could not build name for facility $network,: Invalid argument