Improve use of asset validation errors

[jjnesbitt]

Closes #2540
Closes #1653
Closes #1214

The primary purpose of this PR is to address an issue with asset validation errors for zarr assets. The existing behavior is to share the same status between assets that are in the process of validation, and zarr assets that cannot be validated yet (due to the underlying zarr not being finalized).

This is observed mostly on dandiset 000108, however other dandisets have experienced this as well. This change won't actually address the underlying problem, as the solution to that is simply to finalize the zarrs backing those assets (something I will address separately). However, it will fix the asset validation error that is shown, to more accurately describe the problem.

There are several secondary improvements:

• Remove the deprecated VALIDATING status on the Asset model
• Fix the broken asset validation UI
• Separate out asset validation errors into their own endpoint, removing them from the version info endpoint.
• Enabled by the above improvement, remove the truncation of asset validation errors (originally introduced in Truncate asset validation errors/fix memory usage #2548)

[jjnesbitt]

I still need to address CLI integration...