loader: Fix error message for invalid enum values

Currently if you use --freebsd/toolchain=invalid-value, you get a big
stack trace pointing at this code with the final two lines giving two
copies of the error message intended for the user (along with calling it
and unhaldned exception). Fix this by raising ValueError instead as
caught by the calling code. Also print the enums in kebab-case rather
than snake_case to match the --help output and how we expect users to
provide these values.

Author: jrtc27

pycheribuild/config/loader.py

@@ -100,9 +100,9 @@ def __call__(self, astring: "Union[str, list[str], EnumTy]") -> "Union[EnumTy, l
                     return e
             v = self.enums[enum_value_name]
         except KeyError:
-            msg = ", ".join([t.name.lower() for t in self.enums])
+            msg = ", ".join([t.name.lower().replace("_", "-") for t in self.enums])
             msg = f"{name}: use one of {{{msg}}}"
-            raise argparse.ArgumentTypeError(msg)
+            raise ValueError(msg)
         #       else:
         #           self.action.choices = None  # hugly hack to prevent post validation from choices
         return v