init: make init failures more explicit

This patch proposes to make init failures more explicit.

Before this, the "No init found" message didn't help much.  It could
sometimes be misleading and actually mean "No *working* init found".

This message could hide many different issues:
 - no init program candidates found at all
 - some init program candidates exist but can't be executed (missing
   execute permissions, failed to load shared libraries, executable
   compiled for an unknown architecture...)

This patch notifies the kernel user when a candidate init program is found
but can't be executed.  In each failure situation, the error code is
displayed, to quickly find the root cause.  "No init found" is also
replaced by "No working init found", which is more correct.

This will help embedded Linux developers (especially the newcomers),
regularly making and debugging new root filesystems.

Credits to Geert Uytterhoeven and Janne Karhunen for their improvement
suggestions.

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Janne Karhunen <Janne.Karhunen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>