MdeModulePkg PeiCore: Check error status when processing boot FV

MdeModulePkg PeiCore: Check error status when processing boot FV

Until now the possible errors returned from processing
boot firmware volume were not checked, which could cause
misbehavior in further booting stages. Add relevant assert.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: Jan Dabros <jsd@semihalf.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>