Empty lambda produces poor error message

If the user writes \ -> body, they get this unhelpful error message:

ghci> \ -> ()
<interactive>:1:3: error: parse error on input ‘->’

Furthermore, this is a fatal parse error, so we cannot report any other error in the file.

The fix is easy. Change the following production in the parser:

-         | '\\' apat apats '->' exp
+         | '\\' apats '->' exp

And do the validation in checkFunBind using addError (but not addFatalError, which would prevent error recovery).

While we could also simply allow the nullary lambda (to me, it looks as reasonable as nullary let, e.g. let in ...), that would probably require a proposal. For now, we can continue to reject, but with an improved error message.