[SPARK-6366][SQL] In Python API, the default save mode for save and...

[SPARK-6366][SQL] In Python API, the default save mode for save and saveAsTable should be "error" instead of "append".

https://issues.apache.org/jira/browse/SPARK-6366

Author: Yin Huai <yhuai@databricks.com>

Closes #5053 from yhuai/SPARK-6366 and squashes the following commits:

fc81897 [Yin Huai] Use error as the default save mode for save/saveAsTable.