Enhance error handling for external tool.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Yingke Liu <yingke.d.liu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

git-svn-id: svn://svn.code.sf.net/p/edk2/code/trunk/edk2/BaseTools@16458 6f19259b-4bc3-4df7-8a09-765794883524