BaseTools: Enhance error handling for unsupported toolchain Flags/Path

BaseTools: Enhance error handling for unsupported toolchain Flags/Path

Case1: Cover the Tool PATH is not exist, eg: build MdeModule under GCC5
toolchain and IPF arch.
Case2: Cover the Tool FLAGS is not exist, eg: build OvmfPkg under
CLANG35 toolchain and X64 arch.

fixes: https://bugzilla.tianocore.org/show_bug.cgi?id=595
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>