Previously repmgr would abort with an unhelpful message about being
unable to parse CSV output.
With this commit, it will continue running, and display a list of
inaccessible nodes as an addendum to the main output (unless --csv
or --terse options are specified).
Addresses GitHub #246.
If any issues are detected (e.g. node not reachable, unexpected node status
etc.), "repmgr cluster show" returns exit code 25 ("ERR_NODE_STATUS").
Note that exit code 25 was introduced recently as "ERR_CLUSTER_CHECK",
however it makes sense to use this to indicate issues detected by any
command which can detect node issues.
Addresses GitHub #456.
