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/reboot.cpp b/init/reboot.cpp
index 32816fe..24ccdfc 100644
--- a/init/reboot.cpp
+++ b/init/reboot.cpp
@@ -520,7 +520,7 @@
     auto shutdown_handler = [cmd, command, reboot_target,
                              run_fsck](const std::vector<std::string>&) {
         DoReboot(cmd, command, reboot_target, run_fsck);
-        return 0;
+        return Success();
     };
     ActionManager::GetInstance().QueueBuiltinAction(shutdown_handler, "shutdown_done");