Move /src to /subprojects

This will facilitate breaking up Nix into multiple packages for each
component with Meson.

subprojects/libutil/windows/windows-error.hh

@@ -0,0 +1,51 @@
+#pragma once
+///@file
+
+#include <errhandlingapi.h>
+
+#include "error.hh"
+
+namespace nix::windows {
+
+/**
+ * Windows Error type.
+ *
+ * Unless you need to catch a specific error number, don't catch this in
+ * portable code. Catch `SystemError` instead.
+ */
+class WinError : public SystemError
+{
+public:
+    DWORD lastError;
+
+    /**
+     * Construct using the explicitly-provided error number.
+     * `FormatMessageA` will be used to try to add additional
+     * information to the message.
+     */
+    template<typename... Args>
+    WinError(DWORD lastError, const Args & ... args)
+        : SystemError(""), lastError(lastError)
+    {
+        auto hf = HintFmt(args...);
+        err.msg = HintFmt("%1%: %2%", Uncolored(hf.str()), renderError(lastError));
+    }
+
+    /**
+     * Construct using `GetLastError()` and the ambient "last error".
+     *
+     * Be sure to not perform another last-error-modifying operation
+     * before calling this constructor!
+     */
+    template<typename... Args>
+    WinError(const Args & ... args)
+        : WinError(GetLastError(), args ...)
+    {
+    }
+
+private:
+
+    std::string renderError(DWORD lastError);
+};
+
+}