Error when mixing purity in callback aliases

Also adjust the way we report error while infering to avoid double
reporting errors

internal/compiler/tests/syntax/functions/functions_call.slint

@@ -34,5 +34,5 @@ Xxx := Rectangle {
     }
 
     callback xx <=> foo;
-//                 ^error{Cannot bind to a function}
+//  ^error{Binding to callback 'xx' must bind to another callback}
 }