except/catch: check exceptions list type

Signed-off-by: Julien Danjou <julien@danjou.info>
catch: use the right symbol name in errors

Signed-off-by: Julien Danjou <julien@danjou.info>
compiler: add `else' support in `try'

This is a bit tricky, since we'll also have to support `finally' in the end,
I've introduced an Else statement on my own to be able to recognize it.

This fixes issue #74.

Signed-off-by: Julien Danjou <julien@danjou.info>