Change validation message for expressions

[rherrmannr]

Current behavior:

// this results an error
var x : int16; x = x * x

The error message: Validation: Function __TIMES__ cannot be used here: Return type incompatible: int32 is not subtype of int16

Expected behavior: Replace __TIMES__ with the operator and adapt the error message. For example: Multiply operation cannot be used here. Multiplication results int32 and is not subtype of int16.

This can also be reproduced with other operations, like __PLUS__, __POSTINCREMENT__ and so on.