Improve compiler error message COMPILER-70: The expression cannot be assigned to.

[mrcompator]

Is your feature request related to a problem? Please describe.
I want to call a method from within the constructor. For this the method must be declared as static. Within the method I want to set a variable and so the variable must also be declared static. See example code below.

service SomeExampleService {

    static Boolean running = False;

    construct() {
        runSomething();
    }

    static void runSomething() {

        // just to be on the safe side in case it would run super long
        if (!running) {
            running = True;
            // do lots of stuff here
            running = False;
        }
    }
}

The compiler complains with:
COMPILER-70: The expression cannot be assigned to. ("running")

After talking to Gene and Cam they explained that static in this context also means final.

Describe the solution you'd like
Display a better error message why a value can't be assigned to a static variable. Any new devs coming from Java will very likely encounter the same problem and scratch their heads.
"The expression cannot be assigned to. ("running"). static makes the property final. Use a singleton service instead."

Describe alternatives you've considered
n/a

Additional context
n/a

[ggleyzer]

Currently we have a very generic error production. This is the corresponding place in Expression.java:

public void requireAssignable(Context ctx, ErrorListener errs)
    {
    if (!isAssignable(ctx))
        {
        log(errs, Severity.ERROR, Compiler.ASSIGNABLE_REQUIRED);
        }
    }

I think we could easily add a check for constants (if (isConstant())) and log another, more fitting error. What do you think it should look like? How about this:

"The expression {0} is a constant and cannot be assigned to"