grub-efi.bbclass: Use bb.fatal() instead of raising FuncFailed

grub-efi.bbclass: Use bb.fatal() instead of raising FuncFailed

This sets a good example and avoids unnecessarily contributing to
perceived complexity and cargo culting.

Motivating quote below:

< kergoth> the *original* intent was for the function/task to error via
           whatever appropriate means, bb.fatal, whatever, and
           funcfailed was what you'd catch if you were calling
           exec_func/exec_task. that is, it's what those functions
           raise, not what metadata functions should be raising
< kergoth> it didn't end up being used that way
< kergoth> but there's really never a reason to raise it yourself

FuncFailed.__init__ takes a 'name' argument rather than a 'msg'
argument, which also shows that the original purpose got lost.

(From OE-Core rev: 48c4faa1d7117732974e51428f7ed2f62ad7e7bf)

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>