Output a readable error for workspace root

Signed-off-by: Michał Górny <[email protected]>

pycargoebuild/__main__.py

@@ -25,6 +25,7 @@
 from pycargoebuild.cargo import (
     Crate,
     FileCrate,
+    WorkspaceCargoTomlError,
     get_crates,
     get_package_metadata,
 )
@@ -260,8 +261,16 @@ def repack_crates(tar_out: tarfile.TarFile,
         with f:
             workspace = get_workspace_root(directory)
             crates.update(workspace.crates)
-            pkg_metas.append(
-                get_package_metadata(f, workspace.workspace_metadata))
+            try:
+                pkg_metas.append(
+                    get_package_metadata(f, workspace.workspace_metadata))
+            except WorkspaceCargoTomlError as e:
+                logging.error("The specified directory is a workspace root: "
+                              f"{str(directory)!r}")
+                logging.info(
+                    "Please run pycargoebuild in one of its members: "
+                    f"{' '.join(e.members)}")
+                return 1
     pkg_meta = pkg_metas[0]
 
     if args.no_license:

pycargoebuild/cargo.py

@@ -156,7 +156,10 @@ def with_replaced_license(self, new_license: typing.Optional[str]
 
 class WorkspaceCargoTomlError(RuntimeError):
     """Cargo.toml belongs to a workspace root"""
-    pass
+
+    def __init__(self, members: list[str]) -> None:
+        super().__init__()
+        self.members = members
 
 
 def cargo_to_spdx(license_str: str) -> str:
@@ -226,9 +229,7 @@ def get_package_metadata(f: typing.BinaryIO,
 
     if "package" not in cargo_toml and "workspace" in cargo_toml:
         raise WorkspaceCargoTomlError(
-            "Specified directory seems to be a workspace root, please run "
-            "pycargoebuild on one of its members instead: "
-            f"{' '.join(cargo_toml['workspace']['members'])}")
+            cargo_toml["workspace"]["members"])
 
     pkg_meta = cargo_toml["package"]
     _get_meta_key = functools.partial(get_meta_key,