When converting an mjsunit exception to a string, include the stack

When converting an mjsunit exception to a string, include the stack

Makes debugging failures a bit easier

BUG=none
R=ulan@chromium.org
LOG=n

Committed: https://crrev.com/2e8181acd535f8da6bd14e56f8ded9ef26171e2c
Cr-Commit-Position: refs/heads/master@{#31195}

[jochen (gone - plz use gerrit), 2015-10-09 13:06:02]

ptal

[ulan, 2015-10-09 13:10:24]

lgtm

[jochen (gone - plz use gerrit), 2015-10-09 13:10:50]

The CQ bit was checked by jochen@chromium.org

[commit-bot: I haz the power, 2015-10-09 13:10:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1398703007/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1398703007/1

[commit-bot: I haz the power, 2015-10-09 13:11:57]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-09 13:12:16]

Patchset 1 (id:??) landed as
https://crrev.com/2e8181acd535f8da6bd14e56f8ded9ef26171e2c
Cr-Commit-Position: refs/heads/master@{#31195}