rimport: Improve handling of ValueError.

Author: samsrabin

rimport

@@ -102,15 +102,14 @@ def stage_data(src: Path, inputdata_root: Path, staging_root: Path) -> None:
 
     try:
         rel = src.resolve().relative_to(inputdata_root.resolve())
-    except ValueError:
+    except ValueError as exc:
         if src.resolve().is_relative_to(staging_root.resolve()):
             raise RuntimeError(
                 f"Source file '{src.name}' is already under staging directory '{staging_root}'."
-            )
-        else:
-            raise RuntimeError(
-                f"source not under inputdata root: {src} not in {inputdata_root}"
-            )
+            ) from exc
+        raise RuntimeError(
+            f"source not under inputdata root: {src} not in {inputdata_root}"
+        ) from exc
 
     dst = staging_root / rel
     dst.parent.mkdir(parents=True, exist_ok=True)