Made the build script report failed builds through its exit code (#1067)

app/Build/Tools/CommandOutput.cs

@@ -0,0 +1,12 @@
+namespace Build.Tools;
+
+/// <summary>
+/// What an external tool wrote, and how it ended.
+/// </summary>
+/// <remarks>
+/// The text alone cannot tell whether a tool failed: a build may stop on a diagnostic which no filter
+/// recognizes, or without writing any at all. Only the exit code answers that question reliably.
+/// </remarks>
+/// <param name="Text">Everything the tool wrote to its standard output and its standard error, line by line.</param>
+/// <param name="ExitCode">The exit code of the tool, where anything but zero is a failure.</param>
+public readonly record struct CommandOutput(string Text, int ExitCode);