systemd-boot: improve error message if no previous systemd boot is fo… (#409073)

nixos/modules/system/boot/loader/systemd-boot/systemd-boot-builder.py

···
         available_match = re.search(r"^\((.*)\)$", available_out)
 
         if installed_match is None:
-            raise Exception("could not find any previously installed systemd-boot")
+            raise Exception("Could not find any previously installed systemd-boot. If you are switching to systemd-boot from a different bootloader, you need to run `nixos-rebuild switch --install-bootloader`")
 
         if available_match is None:
             raise Exception("could not determine systemd-boot version")