Better error messages on invalid operators
Summary:
Previously the error message read:
```
Expression trees only support comparison (`<`, `===` etc) and basic arithmetic operators (`+` etc)
```
Change the error to be more specific about what operator we don't support. For some operators, I suggest a possible solution, but since the design of expression trees is so flexible, I do not recommend solutions in other cases.
User reports that:
> Can we improve this error? It took me a minute to realize that != isn't supported comparison.
Reviewed By: Wilfred
Differential Revision:
D28207743
fbshipit-source-id:
541eb8fed3a730160f7e7576d7426e96a3254545