LLVM & OCAML backend: meaningless errors when +Int is used in LHS

[denis-bogdanas]

We need a meaningful error message that helps detecting and fixing the problem.

PR for Ocaml error: #589

Ocaml error:

< File "realdef.ml", line 843, characters 92-93:
< Error: Syntax error: ')' expected
< File "realdef.ml", line 843, characters 76-77:
< Error: This '(' might be unmatched
< [Error] Critical: ocamlopt returned nonzero exit code: 2

[dwightguth]

This should be fixed now.