When a conversion request is cancelled, the state of the previous trajectory is weird

[ascibisz]

Description

If you make an autoconversion request for a decent size file, then cancel the request, and then go back to the viewer from the autoconversion form, the previous trajectory is still there (which is good). However, if the trajectory hasn't been fully played through (aka if all the frames aren't in the cache), the website fails to play any new frames, resulting in a weird user experience.

Expected Behavior

Either:

1. the old trajectory should be fully loaded / able to be interacted with as normal, or
2. the viewer is blank when the user gets back to that page

Note: this octopus ticket is closely related, and will make it so that cancelled autoconversion data never makes it to the website