BaseTools: build: Set ReturnCode on POSTBUILD fail

BaseTools: build: Set ReturnCode on POSTBUILD fail

When build.by POSTBUILD handling section returns other than 0, set
ReturnCode to POSTBUILD_ERROR so build.py exits with return code other
than 0.

Fix for https://bugzilla.tianocore.org/show_bug.cgi?id=1977

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Garrett Kirkendall <garrett.kirkendall@amd.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>