Regenerate thumnails if they can't be found in media store

*This issue was originally created by [**@alistair23**](https://github.com/alistair23) at .*

**Description:**

If Dendrite fails to open a thumbnail with the error `Error generating thumbnails` where `error="open : no such file or directory"` then it should regenerate the thumbnail.

Start by tracing the thumbnail open and generation path that emits `Error generating thumbnails` with `open : no such file or directory`. Reproduce the missing-media-store case and verify that a missing thumbnail is regenerated rather than leaving the error.

Written by the indexing model from the issue text.