fs_mgr: Better error messages in fs_mgr_update_verity_state.

With verity disabled, fs_mgr_update_verity_state spews some confusing
error messages from device-mapper. This change checks whether a device
exists and logs an explicit error.

Bug: N/A
Test: AVB properties are set correctly on AVB device
      Messages are logged correctly with AVB disabled

Change-Id: If490c18cfec2d63ad784972c13ceef63d9aa3e4c