simplified job function to remove the static errors in the return type of the second overload

Author: jurgenvinju

.vscode/launch.json

@@ -38,8 +38,7 @@
             "mainClass": "org.rascalmpl.shell.RascalShell",
             "projectName": "rascal",
             "vmArgs": "-Xss80m -Xmx2g -ea",
-            "console": "integratedTerminal",
-            "args": "--project |cwd:///|" 
+            "console": "integratedTerminal"
         },
         {
             "type": "java",

src/org/rascalmpl/library/util/Monitor.rsc

@@ -100,6 +100,10 @@ with a parameterized workload and the same label as the job name.
 * additional work with ((jobTodo)) is still possible, but you have to repeat the right job label.
 }
 &T job(str label, &T (void (int worked) step) block, int totalWork=1) {
+  if (void (void (int _) _) _ := block) {
+    throw IllegalArgument(block, "`block` argument can not be used by job because it returns `void` and `job` must return something.");
+  }
+
   try {
     jobStart(label, totalWork=totalWork);
     return block(void (int worked) { 
@@ -114,11 +118,6 @@ with a parameterized workload and the same label as the job name.
   }
 }
 
-@synopsis{Captures accidental void closures to throw a better exception than `CallFailed`}
-(&T<:void) job(str label, (&T<:void) (void (int worked) step) block, int totalWork=1) {
-  throw IllegalArgument(block, "`block` argument can not be used by job because it returns `void` and `job` must return something.");
-}
-
 @synopsis{A job block guarantees a start and end, and provides easy access to the stepper interface.}
 @description{
 The convenience function that is passed to the block can be used inside the block to register steps