[MERGE][Bug #196618] Change some assertions to exceptions in LP plugin

[John Arbash Meinel]

John Arbash Meinel has voted approve.
Status is now: Semi-approved
Comment:
Can we get another reviewer? This is pretty trivial, so if we don't get 
one (and I remember) I'll merge this in a couple of days.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C20080303172900.5f01be7b%40lapbert.oxbridgetech%3E