Fuzzer catches too many exceptions

The fuzzer currently does not fail on failing assertions.

Example code (from #4417):
```
pragma solidity ^0.4.24;

contract SomeContract {

  uint public balance = 0;


  function balance(uint number) public {

  }

  function doSomething() public {
    balance(3);
  }
}
```

-- found by @mattaereal 