efi_loader: bootmgr: print a message when loading from BootNext failed

efi_loader: bootmgr: print a message when loading from BootNext failed

If a user defines BootNext but not BootOrder and loading from BootNext
fails, you will see only a message like this:
	BootOrder not defined

This may confuse a user. Adding an error message will be helpful.

An example output looks like this:

=> efidebug boot add 0001 label1 scsi 0:1 "\path1\file1.efi" "--option foo"
=> efidebug boot add 0002 label2 scsi 0:1 "\path2\file2.efi" "--option bar"
=> efidebug boot add 0003 label3 scsi 0:1 "\path3\file3.efi" "--option no"
=> efidebug boot order 0001 0002
=> efidebug boot next 0003
=> bootefi bootmgr
Loading from Boot0003 'label3' failed
Loading from BootNext failed, falling back to BootOrder
Loading from Boot0001 'label1' failed
Loading from Boot0002 'label2' failed
EFI boot manager: Cannot load any image

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Adjust messages.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>