Ensure that the sub-server error string is set to an empty string when
marking a sub-server as running. This will be important for the next
commit, which will retry to start the lnd sub-server even if we error
when starting the lnd-client(s), and therefore may mark the lnd
sub-server as running even after we've errored when starting the
lnd-client(s).
isReadyOverride is a callback that, when set and only if `running` is
not yet true, will be used to determine if a system is ready for a call.
We will pass the request URI to this method along with the
`manualStatus`. The first returned boolean is true if the system should
be seen as ready and the second is true if the override does handle the
given request. If it does not, then we will fall back to our normal
is-ready check.
