Fix error message handling in rt-cli, related to single-user mode

See #4428

rtgui/main-cli.cc

@@ -158,8 +158,10 @@ int main (int argc, char **argv)
 
     try {
         Options::load (quickstart);
-    } catch (Options::Error &) {
+    } catch (Options::Error &e) {
-        printf ("Fatal error!\nThe RT_SETTINGS and/or RT_PATH environment variables are set, but use a relative path. The path must be absolute!\n");
+        std::cerr << std::endl
+                  << "FATAL ERROR:" << std::endl
+                  << e.get_msg() << std::endl;
         return -2;
     }
 

rtgui/options.cc

@@ -81,7 +81,7 @@ bool Options::checkDirPath (Glib::ustring &path, Glib::ustring errString)
         return true;
     } else {
         if (!errString.empty()) {
-            printf ("%s\n", errString.c_str());
+            std::cerr << errString << std::endl;
         }
 
         return false;