Additional UserProxy failed withdraw tests

In UserProxy.sol:

this require:

```solidity
require((wethReceivedPt + wethReceivedYt != 0), "Failed withdraw");
```

is going tobe changed to something like:

```solidity
require((wethReceivedPt != 0 ) && (_amountPT != 0), "Rugged");
require((wethReceivedYt != 0 ) && (_amountYT != 0), "No yield accrued");
```

Tests will need to be written for this.