Ensure backend info is available in exception

Author: abravalheri

src/pyproject_hooks/_in_process/_in_process.py

@@ -70,7 +70,7 @@ def _build_backend():
         try:
             obj = import_module(mod_path)
         except ImportError:
-            msg = "Cannot import {mod_path!r}"
+            msg = f"Cannot import {mod_path!r}"
             raise BackendUnavailable(msg, traceback.format_exc())
 
     if obj_path:

tests/test_call_hooks.py

@@ -34,8 +34,10 @@ def get_hooks(pkg, **kwargs):
 def test_missing_backend_gives_exception():
     hooks = get_hooks("pkg1")
     with modified_env({"PYTHONPATH": ""}):
-        with pytest.raises(BackendUnavailable):
+        msg = "Cannot import 'buildsys'"
+        with pytest.raises(BackendUnavailable, match=msg) as exc:
             hooks.get_requires_for_build_wheel({})
+        assert exc.value.backend_name == "buildsys"
 
 
 def test_get_requires_for_build_wheel():