Return hex representation of UUID in __toString

We were previously outputing a backtrace and dieing because
you can't throw exceptions from __toString.  We wanted that
behavior because at the time we were transitioning from
plain strings to a UUID class and wanted to catch bugs.

We have long since completed that conversion, its now acceptable
to stringify uuid's into hex.

Change-Id: I4039443de39b15f5e67e1f7f2a06298f604ec40b
Merge "Return hex representation of UUID in __toString"