Better error message for missing optional dependencies

[aseyboldt]

If you enable the normalizing flow adaptation, but don't have the required dependencies installed, the error only shows up once it tries to sample in one of the child processes:

RuntimeError: All initialization points failed

Caused by:
    Could not initialize state because of bad initial gradient: Anyhow(ImportError: The 'flowjax' package is required to use normalizing flow adaptation.)

We should check this earlier.