Add error identification

This patch introduces a error member in the deserializer state type to
handle certain parsing errors, such as internal ones (for example
-ENOMEM) and YAJL parsing errors, to be able to tell the plugin what
happened.
Add error identification handling in yajl_null_cb()
Add error identification handling in yajl_boolean_cb()
Add error identification handling in yajl_integer_cb()
Add error identification handling in yajl_string_cb()
Add error identification handling in yajl_map_key_cb()
Merge pull request #468 from matthiasbeyer/add-deserializer_error_handling

Add error identification in deserializer code