Fix error handling

[bgpat]

Type of err was changed in https://github.com/torhve/lua-resty-letsencrypt/commit/5abb0d2a0fab34ca00e78279a5f4061cbc3646b6, but this line has not been changed.

error log says:

[lua] letsencrypt.lua:62: log(): ___***___: Unable to generate cert: letsencrypt.lua:671: attempt to call local 'err' (a table value), context: ssl_certificate_by_lua*, client: 127.0.0.1, server: 0.0.0.0:443