Haskell - generate missing argument error message from either data type

I have the following haskell test code with which I want to test my argument parser for a script.

error' = let mp = runParser AllowOpts globalOptsParser ["-d", "billing"]
          opts = ParserPrefs "suffix" False False False 80
      in fst $ runP mp opts

The required arguments are,

  -d <DB name>
  --sql <SQL SELECT statement>
  --descr <Description>
  --file-path </path/to/file>

I want to test that I get the error message,

Missing: --sql <SQL SELECT statement> --descr <Description>
--file-path </path/to/file>

when I only specify "-d billing".

The above test code gives the following output if I print the result,

Left (MissingError (MultNode [MultNode [MultNode [AltNode [Leaf (Chunk {unChunk = Just --sql <SQL SELECT statement>}),MultNode []]],AltNode [Leaf (Chunk {unChunk = Just --descr <Description>}),MultNode []]],AltNode [Leaf (Chunk {unChunk = Just --file-path </path/to/file>}),MultNode []]]))

Is there a way to generate the expected error message (String) from the above result (Either data type)? Does Haskell provide an obvious function to use for this purpose as I cannot find something in the documentation and googling for examples also didn't produce any answers.