skdiff: return nonzero exit value if there are any mismatches

skdiff: return nonzero exit value if there are any mismatches

This will allow us to replace "gm -r" in our buildbots with "skdiff".  This will speed things up, and work around http://code.google.com/p/skia/issues/detail?id=473 ('PDF gradtext gm image results are nondeterministic')
Committed: https://code.google.com/p/skia/source/detail?r=4097

[epoger, 2012-05-30 19:30:15]

P.S. Tom- I've been sending all these skdiff reviews to you, because you wrote
it originally (as far as I remember) and I figure it's good to mind-meld with
you on the direction of the code... but please let me know if you'd like me to
send the next couple of skdiff reviews to someone else instead.  (And if so, let
me know if you'd like me to CC you on them.)

[(pls use tomhudson at google), 2012-05-31 14:46:46]

LGTM