Fix exception causes in code.py

Author: cool-RR

sphinx/directives/code.py

@@ -227,12 +227,12 @@ def read_file(self, filename: str, location: Tuple[str, int] = None) -> List[str
                     text = text.expandtabs(self.options['tab-width'])
 
                 return text.splitlines(True)
-        except OSError:
-            raise OSError(__('Include file %r not found or reading it failed') % filename)
-        except UnicodeError:
+        except OSError as e:
+            raise OSError(__('Include file %r not found or reading it failed') % filename) from e
+        except UnicodeError as e:
             raise UnicodeError(__('Encoding %r used for reading included file %r seems to '
                                   'be wrong, try giving an :encoding: option') %
-                               (self.encoding, filename))
+                               (self.encoding, filename)) from e
 
     def read(self, location: Tuple[str, int] = None) -> Tuple[str, int]:
         if 'diff' in self.options: