Here is the sentence again:
The comma between milk and cream causes a comma splice. This is a major error. Howard had only a jar of olives and a carton of milk is a complete sentence. Cream of olive soup, Howard decided, would only worsen how he felt is a second complete sentence. A comma is too wimpy a punctuation mark to connect two sentences.
To fix this problem, you could put a period after milk and capitalize the C for cream.