Fix several issues with Differential exception email

Summary:

• Assigning $cc_phids clobbers the correct value assigned on line 80.
• Remove stack trace noise, the trace is always meaningless and well-known.
• Actually show the original body.

Test Plan: Piped mail to the mail receiver and verified the errors didn't CC revision CCs, no longer had traces, and included the original raw text body.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2969