fix: catch more ssh failure modes with help

cli/internal/cli.go

@@ -410,6 +410,24 @@ Good luck!
 
 `
 
+var ServerAddFailMsg = `
+Failed to add server %s.
+
+This could be caused by two things.
+
+Abra isn't picking up your SSH configuration or you need to specify it on the
+command-line (e.g you use a non-standard port or username to connect). Run
+"server add" with "-d/--debug" to learn more about what Abra is doing under the
+hood.
+
+Docker is not installed on your server. You can pass "-p/--provision" to
+install Docker and initialise Docker Swarm mode. See help output for "server
+add"
+
+See "abra server add -h" for more.
+
+`
+
 // SubCommandBefore wires up pre-action machinery (e.g. --debug handling).
 func SubCommandBefore(c *cli.Context) error {
 	if Debug {