When an empty expression is provided in curly brackets inside an f-string we get Syntax Error: "f-string: empty expression not allowed".


This is correct, however error with the same message is raised in the following cases: f"{!foo}", f"{!}", f"{:bar}", f"{=}", etc.
For example, should someone make an error and try to negate an expression with '!' instead of 'not' (i.e. f"{!True}"), they would get a confusing error, saying that the expression is empty, when it isn't. 

I propose we raise an error with the message: "f-string: optional specifier '%c' must follow a non-empty expression.