volume_manager: Move bind/mount failure from ERROR to WARNING

* Prevents users from seeing this message on FBE devices:
  `failed to bind mount /mnt/staging/emulated/media/0 to /storage/emulated`

* FDE users already have the workaround in line 75.

Change-Id: I47d1aaf1a5a85a46fd52dc924a91b378cd800b2d
diff --git a/volume_manager/Utils.cpp b/volume_manager/Utils.cpp
index b8cf4eb..1a0a940 100644
--- a/volume_manager/Utils.cpp
+++ b/volume_manager/Utils.cpp
@@ -187,7 +187,7 @@
 
 status_t BindMount(const std::string& source, const std::string& target) {
     if (::mount(source.c_str(), target.c_str(), "", MS_BIND, NULL)) {
-        PLOG(ERROR) << "Failed to bind mount " << source << " to " << target;
+        PLOG(WARNING) << "Failed to bind mount " << source << " to " << target;
         return -errno;
     }
     return OK;