chain_ex: don't opportunistically add tracebacks to context_ex

chain_ex: don't opportunistically add tracebacks to context_ex

Don't add a traceback to the context_ex while chaining.  This could be
wrong, depending on the context, and in the common case, the traceback
more likely belonged on the result exception, not the context_ex.,
i.e.

  except ... as ex:
      raise chain_ex(ex, pending_ex)

Instead, just combine chain_ex with add_ex_tb when appropriate:

  except ... as ex:
      raise chain_ex(add_tb_ex(ex), pending_ex)

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>