feat: explicitly raise error when trying to use unsupported operation

[knarewski]

Background

GdkPixbuf-based image processing can take over 1GB of memory for processing a single high-resolution photo. We've decided to introduce another processor, hoping that it'll improve Morandi's memory profile and performance.

At the moment a subset of operations has been implemented using vips. Whether the others will be added or deprecated remains to be decided.

Problems

If someone decides to give vips processor a go, but uses unsupported operations, these will currently be silently omitted.

Solution

Explicitly raise error when unsupported operation is requested. User will be able to detect it and act accordingly, eg try to re-render using pixbuf processor.

Notes

Also fixed a little typo which was raising a warning when generating docs (@options -> @option)