fix: say a platform has no hardware targets

The error named platforms/targets/<name>.yml, which read as a file
missing from the repository (finding 63).

scripts/common.py

@@ -664,8 +664,15 @@ def load_target_config(
     targets_dir = os.path.join(platforms_dir, "targets")
     target_file = os.path.join(targets_dir, f"{platform_name}.yml")
     if not os.path.exists(target_file):
+        # The expected state for a platform that ships one build, said as
+        # such: the file path read like a file missing from the repository.
+        with_targets = sorted(
+            Path(f).stem for f in os.listdir(targets_dir)
+            if f.endswith(".yml") and not f.startswith("_")
+        )
         raise FileNotFoundError(
-            f"No target config for platform '{platform_name}': {target_file}"
+            f"{platform_name} has no hardware targets: drop --target "
+            f"(platforms with targets: {', '.join(with_targets)})"
         )
     with open(target_file) as f:
         data = yaml_load(f) or {}

tests/test_tool_messages.py

@@ -168,9 +168,6 @@ class HeldButNotPackedNamesEachReason(unittest.TestCase):
                 "in_pack": False, "omitted": reason, "category": category}
 
     def _printed(self, entries: list[dict]) -> str:
-        import contextlib  # noqa: PLC0415
-        import io  # noqa: PLC0415
-
         out = io.StringIO()
         with contextlib.redirect_stdout(out):
             verify._print_undeclared_section({"undeclared_files": entries}, False)
@@ -219,6 +216,25 @@ class HeldButNotPackedNamesEachReason(unittest.TestCase):
         self.assertIn("Core files held but not packed: 2 (installed with its emulator)", text)
 
 
+class APlatformWithoutTargetsSaysSo(unittest.TestCase):
+    """`--platform misterfpga --target switch` printed 'No target config for
+    platform 'misterfpga': platforms/targets/misterfpga.yml', which reads as
+    a file missing from the repository."""
+
+    def test_the_message_names_the_reason_and_the_platforms_with_targets(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            (Path(tmp) / "targets").mkdir()
+            for name in ("retroarch", "batocera", "_overrides"):
+                (Path(tmp) / "targets" / f"{name}.yml").write_text("targets: {}\n")
+            with self.assertRaises(FileNotFoundError) as caught:
+                common.load_target_config("misterfpga", "switch", tmp)
+        self.assertEqual(
+            str(caught.exception),
+            "misterfpga has no hardware targets: drop --target "
+            "(platforms with targets: batocera, retroarch)",
+        )
+
+
 class OneSpellingPerOption(unittest.TestCase):
     """verify.py -p batocera worked and slots.py -p batocera was refused."""