Non-requested datasets are saved when composites fail to generate

[djhoese]

Describe the bug

Right now the Scene objects save_datasets method defaults to saving all currently loaded/existing datasets. This means that if a composite hasn't been generated yet, either by generate=False or IncompatibleAreas or some unexpected failure then scn.save_datasets() will save dependency datasets that weren't actually requested by the user. This can be very confusing.

To Reproduce

scn = Scene(reader='abi_l1b', filenames=[...])
scn.load(['true_color'])
scn.save_datasets()

Expected behavior

Only true_color would be saved if it exists or it won't be saved. Optionally it could raise an exception if none of the requested datasets (Scene.wishlist) are available to save.

Actual results

C01, C02, C03, etc. geotiffs will be saved.

Screenshots
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Environment Info:

• OS: [e.g. OSX, Windows, Linux]
• SatPy Version: [e.g. 0.9.0]
• PyResample Version:

Additional context
Add any other context about the problem here.