adb: fix apex installation.

Incremental doesn't support apex installation, but we were exiting
instead of returning -1 to fall back to regular installation.

Bug: http://b/151900478
Test: manual
Change-Id: Id27009250090a65fbb45bb65fc39c1799bf1f861
Merged-In: Id27009250090a65fbb45bb65fc39c1799bf1f861
diff --git a/adb/client/adb_install.cpp b/adb/client/adb_install.cpp
index e4d010c..092a866 100644
--- a/adb/client/adb_install.cpp
+++ b/adb/client/adb_install.cpp
@@ -325,7 +325,12 @@
         }
     }
 
-    if (first_apk == -1) error_exit("Need at least one APK file on command line");
+    if (first_apk == -1) {
+        if (!silent) {
+            fprintf(stderr, "error: need at least one APK file on command line\n");
+        }
+        return -1;
+    }
 
     auto files = incremental::Files{argv + first_apk, argv + last_apk + 1};
     if (silent) {