init: use Result<T> for builtin functions

We currently throw out the return values from builtin functions and
occasionally log errors with no supporting context.  This change uses
the newly introduced Result<T> class to communicate a successful result
or an error back to callers in order to print an error with clear
context when a builtin fails.

Example:

init: Command 'write /sys/class/leds/vibrator/trigger transient' action=init (/init.rc:245) took 0ms and failed: Unable to write to file '/sys/class/leds/vibrator/trigger': open() failed: No such file or directory

Test: boot bullhead
Merged-In: Idc18f331d2d646629c6093c1e0f2996cf9b42aec
Change-Id: Idc18f331d2d646629c6093c1e0f2996cf9b42aec
diff --git a/init/builtins.h b/init/builtins.h
index b110f61..f66ae19 100644
--- a/init/builtins.h
+++ b/init/builtins.h
@@ -23,11 +23,12 @@
 #include <vector>
 
 #include "keyword_map.h"
+#include "result.h"
 
 namespace android {
 namespace init {
 
-using BuiltinFunction = std::function<int(const std::vector<std::string>&)>;
+using BuiltinFunction = std::function<Result<Success>(const std::vector<std::string>&)>;
 class BuiltinFunctionMap : public KeywordMap<BuiltinFunction> {
   public:
     BuiltinFunctionMap() {}