✨ Preserve json.JSONDecodeError information when handling invalid J…

…SON in request body, to support custom exception handlers that use its information (#4057)

Co-authored-by: Sebastián Ramírez <tiangolo@gmail.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

fastapi/routing.py

@@ -209,7 +209,9 @@ async def app(request: Request) -> Response:
                         else:
                             body = body_bytes
         except json.JSONDecodeError as e:
-            raise RequestValidationError([ErrorWrapper(e, ("body", e.pos))], body=e.doc)
+            raise RequestValidationError(
+                [ErrorWrapper(e, ("body", e.pos))], body=e.doc
+            ) from e
         except Exception as e:
             raise HTTPException(
                 status_code=400, detail="There was an error parsing the body"