Make Result able to return an error string

luna/Result.h

@@ -2,6 +2,7 @@
 #include <Check.h>
 #include <Move.h>
 #include <PlacementNew.h>
+#include <SystemError.h>
 #include <Types.h>
 
 struct Error
@@ -92,6 +93,12 @@ template <typename T> class Result
         return {m_error};
     }
 
+    const char* error_string()
+    {
+        check(has_error());
+        return ::error_string(m_error);
+    }
+
     T value()
     {
         check(has_value());
@@ -224,6 +231,12 @@ template <> class Result<void>
         return {m_error};
     }
 
+    const char* error_string()
+    {
+        check(has_error());
+        return ::error_string(m_error);
+    }
+
     void value()
     {
         check(has_value());
@@ -242,7 +255,7 @@ template <> class Result<void>
 };
 
 // clang-format off
-#define err Error{0}
+#define err(x) Error{x}
 // clang-format on
 
 #define TRY(expr)                                                                                                      \