buildPythonPackage: better error message if no setuptools is used.

Before packages did not had to set `format = "other";` if had a custom
installPhase and would not produce a dist folder.
The current error message is hard to understand by new users:

> Executing pythonOutputDistPhase
> mv: cannot stat 'dist': No such file or directory

This commit improves the error message.

pkgs/development/interpreters/python/hooks/python-output-dist-hook.sh

@@ -3,7 +3,21 @@ echo "Sourcing python-catch-conflicts-hook.sh"
 
 pythonOutputDistPhase() {
     echo "Executing pythonOutputDistPhase"
-    mv "dist" "$dist"
+    if [[ -d dist ]]; then
+        mv "dist" "$dist"
+    else
+        cat >&2 <<EOF
+The build contains no ./dist directory.
+If this project is not setuptools-based, pass
+
+  format = "other";
+
+to buildPythonApplication/buildPythonPackage or another appropriate value as described here:
+
+  https://nixos.org/manual/nixpkgs/stable/#buildpythonpackage-function
+EOF
+        false
+    fi
     echo "Finished executing pythonOutputDistPhase"
 }