Improve error messages for save_to_hive_table

when save_to_hive_table fails because

• invalid JDBC URI

• missing auth for KRB connection

the error message is just generic 500 error with no details, forward the error message from JDBC connection to the user

also update docs to mention that KRB details are necessary in JDBC connection

http://docs.h2o.ai/h2o/latest-stable/h2o-docs/data-munging/downloading-data.html?highlight=save_to_hive#save-as-a-hive-table