init: add [[nodiscard]] to Result

It's better to either check these results or explicitly ignore them.
Only a few callers weren't already doing this, so it's relatively
trivial to enforce.

Test: build
Change-Id: I44cdc342e46128f66cac914aaa0b9b4559cacd8c
diff --git a/init/result.h b/init/result.h
index fc03962..0e3fd3d 100644
--- a/init/result.h
+++ b/init/result.h
@@ -151,7 +151,7 @@
 }
 
 template <typename T>
-class Result {
+class [[nodiscard]] Result {
   public:
     Result() {}
 
@@ -170,6 +170,8 @@
         : contents_(std::in_place_index_t<1>(), std::move(result_error.error_string),
                     result_error.error_errno) {}
 
+    void IgnoreError() const {}
+
     bool has_value() const { return contents_.index() == 0; }
 
     T& value() & { return std::get<0>(contents_); }