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

Review URL: https://codereview.chromium.org/1179523002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#333760}

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