EHS default to series poster if season poster missing

[bigjohn 753]

Any chance the EHS could default to showing the series poster (primary) when the season poster is not available? I frequently come across shows that don't have season posters when they are getting started on a new season, EHS just shows the blank folder image.

[ebr 16169]

This might have to be handled on a per-theme basis to limit its scope and avoid screwing up other displays...or maybe not.  I'll have to look at it.