git stash: Give friendlier errors when there is nothing to apply

git stash: Give friendlier errors when there is nothing to apply

The change makes sure a stash (given or default) exists before
checking if the working tree is dirty.

If the default stash is requested, the old message was scary and
included a 'fatal' error from rev-parse:
     fatal: Needed a single revision
     : no valid stashed state found

It is replaced with a friendlier 'Nothing to apply' error, similar to
'git stash branch'.

If a specific stash is specified, the 'Needed a single revision' errors
from rev-parse are suppressed.

Signed-off-by: Ori Avtalion <ori@avtalion.name>
Acked-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>