Exceptions in callbacks are not handled properly

Exceptions occurring in the evaluation or progress reporting callbacks currently cause a print to `stderr` and an `LBFGSError` because the wrong value is returned from the callback:

```
Exception UnboundLocalError: "local variable 'f_calls' referenced before assignment" in 'lbfgs._lowlevel.call_eval' ignored
```

The exception should be passed up instead.
