Fix dialyzer reporting unmatched_return for Sentry.PlugCapture

[lowang]

Hello

I've got a project with stricter dialyzer settings:

dialyzer: [
        ...
        flags: [
          :error_handling,
          :race_conditions,
          :underspecs,
          :unmatched_returns
        ]
      ],

and for Phoenix Endpoint module I've got warning:

lib/app_web/endpoint.ex:1:unmatched_return
The expression produces a value of type:

  :excluded
  | :ignored
  | :unsampled
  | {:error, :invalid_dsn | {:invalid_json, _}}
  | {:ok, <<>> | %Task{:owner => pid(), :pid => nil | pid(), :ref => reference()}}

but this value is unmatched.

I cannot silence use Sentry.PlugCapture for dialyzer the usual way with _ =, only dialyzer.ignore-warnings can shut it.
That happens because Sentry.capture_exception is returning multiple values.

Would you accept fix like that?
Another approach would be to have capture_exception function that returns just single atom but I'm not sure what would be a right name for it.

[mitchellhenke]

Thanks for the pull request!