Move monitor init to it's own method and refactor config validate to return err

test/invalid-config-wrong-hcl-type.hcl

@@ -0,0 +1,12 @@
+check_interval = "1s"
+
+alert "log_command" {
+  command = "should be a list"
+}
+
+monitor "Command" {
+  command =  ["echo", "$PATH"]
+  alert_down =  ["log_command"]
+  alert_every =  2
+  check_interval =  "10s"
+}