deal with read errors from s3 while inferring encoding

we sometimes get errors reading from s3 trying to infer encoding.
specifically, 'content-length does not match' errors from the aws-sdk
gem.  we're trying to upgrade the gem to improve those cases, but we
can at least not crash the job.

fixes CNVS-15442

test plan:
- attachment uploads to s3 should still work

Change-Id: Ie9dcc98098d9a5b3555398859476276fcacad79b
Reviewed-on: https://gerrit.instructure.com/40938
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>