fix compiler crash when attempting to call a non-function

compiler/res/version.txt

@@ -1 +1 @@
-7.5
+7.6-dev

compiler/src/prog8/compiler/astprocessing/AstIdentifiersChecker.kt

@@ -154,7 +154,7 @@ internal class AstIdentifiersChecker(private val errors: IErrorReporter,
                 }
             }
             null -> {}
-            else -> throw FatalAstException("weird call target")
+            else -> errors.err("cannot call this as a subroutine or function", call.target.position)
         }
     }
 }

examples/test.p8

@@ -4,8 +4,8 @@
 main {
     sub start() {
 
-        ubyte @shared xx = @(cx16.r5)
-        xx++
+        uword foobar
 
+        foobar()
     }
 }