Elements should be validated before starting to write

[timtreis]

Is your feature request related to a problem? Please describe. I am writing a large sdata object I created to disk. It wrote the image for 12 min and then failed because my adata.obs[region_key] was not categorical.

Describe the solution you'd like All the validations should happen before we start writing.

[LucaMarconato]

I totally agree, thanks for reporting. It may be fixed by https://github.com/scverse/spatialdata/pull/703, I will check this when merging that PR.