How to assign the output of a command to a variable while checking if the command was successful?

I'm going to try my best to explain the question. Please don't hesitate to ask for clarification if needed.

Ultimately, I want

1. to assign the output of a command to a variable while
2. maintaining the possibility to check if the command was successful or not.

For the first part, normally, I would do something like this:

{
  ls nonexistingfolder
} && {
  echo "fine"
} || {
  echo "error"
}

However, above code doesn't work, when I'm using export to save the output to a variable:

{
  export MESSAGE=$(ls nonexistingfolder)
} && {
  echo "fine"
} || {
  echo "error"
}

Above code will always print fine.

I tried something with an if statement:

export MESSAGE=$(ls notexistingfolder)
if [ -z "$MESSAGE" ] ; then echo "error" ; else echo "fine" ; fi

But above code won't work when the command succeeds without output (for example if ls lists an empty folder):

export MESSAGE=$(ls emptyfolder)
if [ -z "$MESSAGE" ] ; then echo "error" ; else echo "fine" ; fi

Above code falsely prints error.

So I thought about defining an "error value" for $MESSAGE:

export MESSAGE=$(ls notexistingfolder || echo "error")
if [ "$MESSAGE" = "error" ] ; then echo "error" ; else echo "fine" ; fi

Above code prints error as it should.

export MESSAGE=$(ls emptyfolder || echo "error")
if [ "$MESSAGE" = "error" ] ; then echo "error" ; else echo "fine" ; fi

Above code prints fine as it should.

However, there is the low possibility that the command will succeed and return a string which matches my "error value" exactly (for example if ls lists a folder which contains a folder called error):

export MESSAGE=$(ls folderwitherrorfolder || echo "error")
if [ "$MESSAGE" = "error" ] ; then echo "error" ; else echo "fine" ; fi

Above code will falsely print error.

What can I do? Thank you! :-)