adb: treat non-successful inc-server runs as failures

This way if an unknown device returns anything but a clear
success report from an incremental installation adb would retry
non-incrementally

Bug: 158006039
Test: manual install on R and Q
Change-Id: I9950273bd60ba75fe12a3706d2ccbee136987d13
diff --git a/adb/client/incremental.cpp b/adb/client/incremental.cpp
index 1e985ed..3033059 100644
--- a/adb/client/incremental.cpp
+++ b/adb/client/incremental.cpp
@@ -180,7 +180,7 @@
     int print_fds[2];
     if (adb_socketpair(print_fds) != 0) {
         if (!silent) {
-            fprintf(stderr, "Failed to create socket pair for child to print to parent\n");
+            fprintf(stderr, "adb: failed to create socket pair for child to print to parent\n");
         }
         return {};
     }
@@ -207,10 +207,15 @@
     Result result = wait_for_installation(pipe_read_fd);
     adb_close(pipe_read_fd);
 
-    if (result == Result::Success) {
-        // adb client exits now but inc-server can continue
-        serverKiller.release();
+    if (result != Result::Success) {
+        if (!silent) {
+            fprintf(stderr, "adb: install command failed");
+        }
+        return {};
     }
+
+    // adb client exits now but inc-server can continue
+    serverKiller.release();
     return child;
 }