This simplifies some logic and presents more directly useful error
messages as well as firming up the `osutil.get_os_id` contract with
docs and a now-uniform return protocol.

The `test_support_url_multi` test is also simplified to remove the
ineffective dedup test and is sped up and stabilized by switching a real
bintray protoc fetch out for a local file url fetch of a small test
file.

 src/python/pants/binaries/binary_util.py             | 44 +++++++++++++++++---------------------
 src/python/pants/util/osutil.py                      |  8 ++++++-
 tests/python/pants_test/binaries/BUILD               |  2 ++
 tests/python/pants_test/binaries/test_binary_util.py | 57 +++++++++++++++++++++++++++++---------------------
 4 files changed, 61 insertions(+), 50 deletions(-)

Now locally green on every run vs ~15% of the time and much faster:

./pants test tests/python/pants_test/binaries/:
CI went green here:

  https://travis-ci.org/pantsbuild/pants/builds/146512692