OK, given that, we just need to consider 3 cases, where last year's earnings are positive, zero, and negative.
If last year's earnings were positive, your A149/A150-1 formula works fine to calculate the percentage increase/decrease.
If last year's earnings were 0, then you can't calculate any kind of reasonable percentage increase. If you start with 1 dollar, and double it to 2 dollars, you can say you're 2 times better (100% better). But if you go from 0 to 1, how many times better are you? 1? 10? 1,000,000? It's equivalent to dividing by zero, which is a no-no.
Now if last year's earnings were negative, you still face a similar problem, -1 to 1 means you're -1 times better? Nonsensical. However, if you want to say that OK, I lost 1 dollar last year, but I made twice as much as I lost year this year, so 200% improvement (not strictly mathematically correct, but I suppose you can get something out of it), then you can use the same formula, but you'll need to multiply the result by -1.
Given that, you can combine those 3 cases into one formula:
=IF(A150=0,"No percentage comparison possible",(A149/A150-1)*SIGN(A150))
Hope this makes sense!