print error when given non-extensible config

that describes the issue (inability to mix in nixos-shell's modules) and
links to the README

Author: tomeon

bin/nixos-shell

@@ -65,7 +65,8 @@ if [[ -z "$flake_uri" ]]; then
   )
 else
   extraBuildFlags+=(
-     --extra-experimental-features "flakes"
+    --extra-experimental-features "flakes"
+    --argstr flakeStr "$flake"
     --argstr flakeUri "$flake_uri"
     --argstr flakeAttr "${flake_attr:-"vm"}"
   )

share/nixos-shell.nix

@@ -2,6 +2,7 @@
 , system ? builtins.currentSystem
 , configuration ? <nixos-config>
 
+, flakeStr ? null # flake as named on the command line
 , flakeUri ? null
 , flakeAttr ? null
 }:
@@ -42,8 +43,28 @@ if flakeUri != null then
   if flakeSystem != null then
     if flakeSystem ? "extendModules" then
       flakeSystem.extendModules { modules = nixosShellModules; }
-    else
+    else if flakeSystem ? "override" then
       flakeSystem.override (attrs: { modules = attrs.modules ++ nixosShellModules; })
+    else
+      throw ''
+          '${flakeStr}#${flakeAttr}' is missing the expected 'override' attribute.
+
+          Please ensure that '${flakeStr}#${flakeAttr}' is an overridable attribute set by declaring it with 'lib.makeOverridable'.
+
+          For instance:
+
+              nixosConfigurations = let
+                lib = nixpkgs.lib;
+              in {
+                "${flakeAttr}" = lib.makeOverridable lib.nixosSystem {
+                  # ...
+                };
+              };
+
+          Alternatively, upgrade to a version of nixpkgs that provides the 'extendModules' function on NixOS system configurations.
+
+          See https://github.com/Mic92/nixos-shell#start-a-virtual-machine for additional information.
+      ''
   else if flakeModule != null then
     mkShellSystem flakeModule
   else