BaseTools: Enhance error message when file is not exist for Gensec

When the file is not exist in workspace or packages path, current
Gensec tool doesn't report exactly error message.

FILE FV_IMAGE = 11111111-4CF1-42D8-A0C3-B3F60779dF4D  {
  SECTION GUIDED A7717414-C616-4977-9420-844712A735BF {
    SECTION FV_IMAGE = TestPkg/Test.fd
  }
}

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Yonghong Zhu <yonghong.zhu@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>