IMPALA-1185: Make Avro and Seq writers unsupported

Avro and Sequence writers are only available if query option
ALLOW_UNSUPPORTED_FORMATS is set to true, prints an error otherwise.

Change-Id: I597039f7c68f708fda10f848531eb557d6910f92
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4539
Reviewed-by: Nong Li <nong@cloudera.com>
Tested-by: jenkins