BaseTools/BinToPcd: Clarify error message for --type HII

BaseTools/BinToPcd: Clarify error message for --type HII

https://bugzilla.tianocore.org/show_bug.cgi?id=963

Update error message for --type HII.  If either --variable-guid
or --variable-name is missing, then print an error message that
states that both --variable-guid and --variable-name are required.

Cc: Yanyan Sun <yanyan.sun@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>