**TL;DR**: Parsing stderr on the fly can be complicated, so simplify it by storing stderr into variable and operate variable contents later.

What you may want to consider doing is capturing `stderr` to variable, and afterwards checking if it's not empty. Consider the example below:

    $ foo=$(stat /etc/passwd 2>&1  > /dev/tty )
      File: /etc/passwd
      Size: 1710      	Blocks: 8          IO Block: 4096   regular file
    Device: fd00h/64768d	Inode: 537594      Links: 1
    Access: (0644/-rw-r--r--)  Uid: (    0/    root)   Gid: (    0/    root)
    Access: 2019-03-08 15:28:10.481688274 -0700
    Modify: 2019-03-08 15:28:10.333669255 -0700
    Change: 2019-03-08 15:28:10.341670283 -0700
     Birth: -
    $ test "x$foo" = "x" && echo "empty"

Command substitution `$(...)` runs a subshell and captures `stdout` only, hence we want to duplicate the file descriptor 1 onto 2 `2>&1` so that stderr now can be captured by command substitution, however we still want to see normal output on screen, hence afterwards we point `stdout` at the controlling terminal `/dev/tty`.

Now, what if we actually capture stderr into the variable ?

    $ foo=$(stat nonexistent.txt 2>&1  > /dev/tty )
    $ test "x$foo" = "x" && echo "empty" || echo "not empty"
    not empty
    $ printf "<Arbitrary Text>%s\n" "$foo"
    <Arbitrary Text>stat: cannot stat 'nonexistent.txt': No such file or directory

As you can see the because `stderr` is captured we can wrap arbitrary text around it as we wish.  The main reason for using a variable is that standard shell methods such as pipeline and command substitution operate on `stdout` file descriptor 1, so there's not a lot we can do to parse `stderr` on the fly. Of course, we could use a named pipe and redirect to it as `stat foobar.txt 2> /tmp/named_pipe.fifo`, but the problem with that is blocking - information sent to pipes is buffered until consumed by another process, so your shell script will be stuck. Of course, this could be handled via starting a background process, but IMHO it's more complexity than necessary, and doing multiple processes is far better done in a programming language like Python where you can actually directly access multiprocessing calls. 

Side note: `test` and `[` are the same command, so you could just as well write `[ "x$foo" = "x" ]` and use that in a more elaborate if-else statement