css: Emit a special error for a common error

css: Emit a special error for a common error

This CSS:
  calc(5px+3px)
is wrong because it gets broken to:
  calc(  5px  +3px  )
which is 2 numbers inside the calc, and what you want is:
  calc(  5px  +  3px  )
but you need to add a space to get this, like so:
  calc(5px + 3px)
which is the recommended way to write calc() statements.

So whenever we encounter an error, check if the next token is a signed
number and if so, include it in the error message.