[pinpoint] Change Job.exception from a StringProperty to a TextProperty.
[pinpoint] Change Job.exception from a StringProperty to a TextProperty.

A StringProperty is indexed, and can be up to 1500 bytes long. A TextProperty is
unindexed and has unlimited length. That's what we want.

Review-Url: https://chromiumcodereview.appspot.com/3008303002

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