Report a simulator that crashes or closes right after the start

The exit code, read with its signal or NTSTATUS, and the last lines of the
error stream - or of log.txt when it stayed silent. The stream is captured
only while the starter stays to read it.

StarterNG/Application/Abstractions/IProcessLauncher.cs

@@ -8,13 +8,24 @@ public interface IProcessHandle
 {
     bool HasExited { get; }
 
+    /// <summary>Once it has ended; null while running, or for a process the starter did not start.</summary>
+    int? ExitCode { get; }
+
+    /// <summary>The last lines of the error stream, when it was captured.</summary>
+    IReadOnlyList<string> ErrorTail { get; }
+
     Task WaitForExitAsync(CancellationToken cancellationToken = default);
 }
 
 public interface IProcessLauncher
 {
+    /// <summary>
+    /// Starts a process. With captureErrors its error stream is kept for the
+    /// handle - only for a process the starter outlives: a write to a pipe nobody
+    /// reads any more can kill the writer.
+    /// </summary>
     IProcessHandle? Start(string executablePath, IReadOnlyList<string> arguments, string? workingDirectory,
-                          out string? error);
+                          bool captureErrors, out string? error);
 
     bool OpenInShell(string pathOrUrl);