Add require check on PriceOracle.addTokenPair for non-zero price

[zareth-san]

Describe the bug
If a newly added token pair's oracle returns a price of 0 inside of PriceOracle.addTokenPair, then that token pair will never be useable going forward.

To Reproduce
Steps to reproduce the behavior:

1. Call PriceOracle.addTokenPair using an oracle that returns a price of 0.

Expected behavior
I expect any call to PriceOracle.addTokenPair will not render that token pair useless

Additional context
To fix this, we should require a check in PriceOracle.addTokenPair to make sure we never set a price of 0, or else we'll never be able to use that token pair again.