[merge request] Librep: better message on process exec failure.

[Teika Kazura <teika lavabit com>]

Please merge my librep branch. Librep now emits better message on
process execution failure.

I was flooded by tons of messages:
 child subprocess can't exec: No such file or directory
But it didn't give me any clue. ;/ With this patch, I'm happy.

It includes minor doc improvements as usual.

Regards,
Teika (Teika kazura)