Re-raise the last exception in the current scope.

`raise` with no arguments finds the last exception in the active scope; this
is the correct way to re-raise an exception that is caught but not handled.
Raising the caught exception object obscures stack information (like line
number) that may be useful for debugging.
Merge pull request #1796 from whilp/re-raise

Re-raise the last exception in the current scope. Fixes #1796.