Check the injected error type available before write error type

before:
0: llc      vaddr = 0x7fee6b865400 paddr = f90e0eb400
./einj_mem_uc: write error on '/sys/kernel/debug/apei/einj/error_type'

after:
0: llc      vaddr = 0x7f86e6bac400 paddr = f915477400
./einj_mem_uc: no support for error type: 0x2

[Tony: re-word error message]

Signed-off-by: Liu Xinpeng <liuxp11@chinatelecom.cn>
Signed-off-by: Tony Luck <tony.luck@intel.com>