Make installd log A/B missing artifacts

In the case of A/B update failures it can be difficult to determine
where things went wrong. This change explictly logs missing files to
provide more information for investigators

Test: treehugger
Bug: 181182967
Change-Id: Ifa27d2b9640270e3b250e6e96ddf7681fb1f519d
diff --git a/cmds/installd/dexopt.cpp b/cmds/installd/dexopt.cpp
index 4dfd1d0..0cf50a3 100644
--- a/cmds/installd/dexopt.cpp
+++ b/cmds/installd/dexopt.cpp
@@ -2112,8 +2112,9 @@
     {
         struct stat s;
         if (stat(b_path.c_str(), &s) != 0) {
-            // Silently ignore for now. The service calling this isn't smart enough to understand
-            // lack of artifacts at the moment.
+            // Ignore for now. The service calling this isn't smart enough to
+            // understand lack of artifacts at the moment.
+            LOG(VERBOSE) << "A/B artifact " << b_path << " does not exist!";
             return false;
         }
         if (!S_ISREG(s.st_mode)) {