Consumers of OutputFormatter::formatProto should handle NH exceptions

When an execution results in a dataset that isn't able to provide all required fields to produce a valid instance of the requested output format, it's reasonable to throw.
Consumers [1] are currently not expecting this, and should be modified to catch those and handle them.

[1]
https://github.com/envoyproxy/nighthawk/blob/481ecbe4ec03184c19c630bbef8a97a6c2d11e8a/source/client/service_impl.cc#L42
https://github.com/envoyproxy/nighthawk/blob/481ecbe4ec03184c19c630bbef8a97a6c2d11e8a/source/client/client.cc#L55