Add line break in default error formatter

This is related to a previous PR I submitted, #36, but better I think. And this time it's in bandit code, not in snowhouse.

Before:

```
My object has some behavior:
/some/very-long/path/to/the-test.cc:154: Expected: equal to #<value-1>
Actual: #<value-2>
```

After:

```
My object has some behavior:
/some/very-long/path/to/the-test.cc:154:
Expected: equal to #<value-1>
Actual: #<value-2>
```