Error position for "Function not found" on operators points to first operand

[kim8823]

This is just a cosmetic issue and not even strictly wrong. The part of the source text that forms the function-call expression does indeed start there, but it would still be nice if the error actually pointed at the operator symbol.

Come to think about it, the wording could also be improved to say "Operator" and maybe even show the signature as infix syntax. The current message might be pretty confusing to rhai-only users that have nothing to do with embedding the engine.

- Function not found: * (map, i64)
+ Operator not found: map * i64