Return number of failing NUnit tests

### What You Are Seeing?

When running NUnit Tests from Cake and any test fails Cake throws an exception. This is due to the way return codes are handled by the [NUnit3Runner](https://github.com/cake-build/cake/blob/develop/src/Cake.Common/Tools/NUnit/NUnit3Runner.cs):

```csharp
if (exitCode <= 0)
{
switch (exitCode)
{
// handle other stuff, not relevant
}
}
else
{
error = string.Format(CultureInfo.InvariantCulture, "{0} test(s) failed", exitCode);
}

const string message = "{0}: {1} (exit code {2}).";
throw new CakeException(string.Format(CultureInfo.InvariantCulture, message, GetToolName(), error, exitCode));
```

At first this makes sense, since the return code != 0 indicates no success. But when I use the [NUnit3Settings](https://cakebuild.net/api/Cake.Common.Tools.NUnit/NUnit3Settings/) with `StopOnError = false` I would expect not only the NUnit console runner to continue when a test failed but also the Cake [NUnit3Runner](https://github.com/cake-build/cake/blob/develop/src/Cake.Common/Tools/NUnit/NUnit3Runner.cs).

Why? With the NUnit console runner any return code above zero describes the number of failing tests, and is not an indication of something that "failed" in a technical sense, e.g. when I run tests and set `StopOnError = false` I expect to be the one responsible for handling failures (in my case, reporting them to TeamCity or writing a playlist file) - I don't expect the need of catching an exception, and then again deciding on my own if I can handle the exception (return code > 0) or I have to re-throw the exception.

### What is Expected?

I expect that the [NUnit3Runner](https://github.com/cake-build/cake/blob/develop/src/Cake.Common/Tools/NUnit/NUnit3Runner.cs) doesn't throw an exception for return codes above zero when I use `StopOnError = false` while running NUnit tests.
I'd also love to have access to the test results using the [NUnit3Result](https://cakebuild.net/api/Cake.Common.Tools.NUnit/NUnit3Result/) (I know it contains the path to the result XML and some way of being able to transform the XML, but I'm talking about a list of for example `NUnit3TestCaseResult` (I just totally made this class up on the fly) which contains the name of the test, failure message etc - A deserialized representation of the NUnit result XML file, basically).

Start in src/Cake.Common/Tools/NUnit/NUnit3Runner.cs and review how NUnit3Settings.StopOnError and NUnit3Result are used. Trace the NUnit exit-code handling and result XML path, then define completion as handling positive failing-test counts without the unwanted exception when StopOnError is false and exposing the requested result data if that scope is retained.

Written by the indexing model from the issue text.