tree-wide: correct cases where return log_{error,warning} is used without value

In various cases, we would say 'return log_warning()' or 'return log_error()'. Those
functions return 0 if no error is passed in. For log_warning or log_error this doesn't
make sense, and we generally want to propagate the error. In the few cases where
the error should be ignored, I think it's better to split it in two, and call 'return 0'
on a separate line.

src/resolve/resolved-manager.c

@@ -350,7 +350,8 @@ static int determine_hostname(char **full_hostname, char **llmnr_hostname, char
 #if HAVE_LIBIDN2
         r = idn2_to_unicode_8z8z(label, &utf8, 0);
         if (r != IDN2_OK)
-                return log_error("Failed to undo IDNA: %s", idn2_strerror(r));
+                return log_error_errno(SYNTHETIC_ERRNO(EUCLEAN),
+                                       "Failed to undo IDNA: %s", idn2_strerror(r));
         assert(utf8_is_valid(utf8));
 
         r = strlen(utf8);