fix(agent): cleaning up models that fail to load

[driev]

If an error occurred whenever a model was loading, the artefacts belonging to the model would remain on disk in the model directory, potentially causing the disk to fill up unnecessarily.

This PR adds a cleanup function, which is executed whenever an error that leads to a LOAD_FAILED event occurs.

[CLAassistant]

All committers have signed the CLA.

[lc525]

The changes lgtm.

If in the future we end up with more places where we return errors on LoadModel perhaps we'll move to a defer function with a named return value, where we test if the error is not null and do the cleanup (I've seen that in the initial commit you also had a named returned value).

[driev]

The changes lgtm.

If in the future we end up with more places where we return errors on LoadModel perhaps we'll move to a defer function with a named return value, where we test if the error is not null and do the cleanup (I've seen that in the initial commit you also had a named returned value).

Agreeed - a defer function was the inital approach, but this seems tidier for now due to: https://github.com/SeldonIO/seldon-core/pull/5857/files#diff-9b56b9f193a85a84314608136ea3c0f5eb0f64346b70c81ebef3481fccf7ba21R618.