improve error reporting for lookahead in macros

[lsf37]

See #1088 -- instead of just a syntax error we should be getting an explanation.

[lsf37]

Alternatively: allow lookahead in macros and at any nesting level syntactically, but add a check in the semantic checking phase that rules out expressions where lookahead is not used at the top level.

[michab66]

That alternative option would be great. :)