Ability to fail test when seed corpus cannot be parsed

I would like our fuzz test target to fail when the input in the seed corpus is malformed.

Currently, when we get the following error, the tests still pass without doing anything:

```
[libprotobuf ERROR external/com_google_protobuf/src/google/protobuf/text_format.cc:309] Error parsing text-format ...
```

This would make it easier to maintain if someone changes the proto structure without changing the seed corpus.