Confusing error when media_reference is missing

If you have a Clip without a media_reference, then you get this error when reading the file:
```
$ otiostat clip_missing_media_reference.otio
There was a system error: "While reading object named '' (of type 'opentimelineio::v1_0::Clip'): media_reference (near line 4583)"
```

There are two enhancements we could do here:
1. Make the error message better, for example: "Clip is missing required property 'media_reference' (near line 4583)"
2. Perhaps we could silently allow this, treating the media_reference as null, or MissingReference.

Here is a simple example:
[clip_missing_media_reference.otio.zip](https://github.com/PixarAnimationStudios/OpenTimelineIO/files/4949217/clip_missing_media_reference.otio.zip)

Start by reproducing the failure with otiostat and the attached clip_missing_media_reference.otio example. Trace how the missing media_reference is reported while reading a Clip, then review the two proposed behaviors in the issue and confirm the selected behavior with a regression test. Done means the missing-property case has clear, intentional behavior and the confusing error is no longer produced.

Written by the indexing model from the issue text.