NaCl: Clean up error handling during NaCl loader startup in OnStart()

NaCl: Clean up error handling during NaCl loader startup in OnStart()

If handling of NaClProcessMsg_Start fails, we want the NaCl loader
process to exit, otherwise it will potentially hang around uselessly
without being killed.

So change the error handling to exit with fatal errors instead of
returning to the message loop.

nonsfi/nonsfi_listener.cc already uses fatal errors, so no change is
required there.

BUG=496287
TEST=none

Committed: https://crrev.com/2127015393cf11f159905b0232f01e5b5310ed41
Cr-Commit-Position: refs/heads/master@{#333760}

[Mark Seaborn, 2015-06-10 16:58:05]

mseaborn@chromium.org changed reviewers:
+ jvoung@chromium.org

[jvoung (off chromium), 2015-06-10 17:25:20]

lgtm

[Mark Seaborn, 2015-06-10 17:29:04]

The CQ bit was checked by mseaborn@chromium.org

[commit-bot: I haz the power, 2015-06-10 17:30:02]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1179523002/1

[commit-bot: I haz the power, 2015-06-10 18:14:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-06-10 18:14:49]

Patchset 1 (id:??) landed as
https://crrev.com/2127015393cf11f159905b0232f01e5b5310ed41
Cr-Commit-Position: refs/heads/master@{#333760}