[HOC][Poetry] Small tweaks for validation code

[ajpal]

A couple things came up while I was doing the last pass of validation code on levelbuilder:

1. We need to set validationInfo.backgroundEffect when the background image block is used so that it counts as having a background effect for validation code
2. Setting validationInfo.endFrame to POEM_DURATION means that the level ends as soon as the poem ends, but it looks better to have some buffer time. Setting it to POEM_DURATION * 1.25 will feel more natural
3. Slight adjustment to validation strings for level 7