journalctl: improve error messages

Follow-up for #12230.

src/journal/journalctl.c

@@ -1949,11 +1949,13 @@ static int simple_varlink_call(const char *option, const char *method) {
 
         r = varlink_connect_address(&link, "/run/systemd/journal/io.systemd.journal");
         if (r < 0)
-                return log_error_errno(r, "Failed to connect to journal: %m");
+                return log_error_errno(r, "Failed to connect to /run/systemd/journal/io.systemd.journal: %m");
+
+        (void) varlink_set_description(link, "journal");
 
         r = varlink_call(link, method, NULL, NULL, &error, NULL);
         if (r < 0)
-                return log_error_errno(r, "Failed to execute operation: %s", error);
+                return log_error_errno(r, "Failed to execute varlink call: %s", error);
 
         return 0;
 }

src/shared/varlink.c

@@ -228,10 +228,15 @@ static inline const char *varlink_server_description(VarlinkServer *s) {
 
 static void varlink_set_state(Varlink *v, VarlinkState state) {
         assert(v);
+        assert(state >= 0 && state < _VARLINK_STATE_MAX);
 
-        varlink_log(v, "varlink: changing state %s → %s",
-                  varlink_state_to_string(v->state),
-                  varlink_state_to_string(state));
+        if (v->state < 0)
+                varlink_log(v, "varlink: setting state %s",
+                            varlink_state_to_string(state));
+        else
+                varlink_log(v, "varlink: changing state %s → %s",
+                            varlink_state_to_string(v->state),
+                            varlink_state_to_string(state));
 
         v->state = state;
 }