Kick off 0.87 with better callstack underflow handling

TODO.FACTOR.txt

@@ -38,7 +38,6 @@
 - allow rebinding styles
 - fix windows gcc issue
 - robustify stepper -- see if step back past a throw works
-- empty callstack: should throw an exception instead of being a critical error
 
 + ui:
 

library/version.factor

@@ -1,2 +1,2 @@
 IN: kernel
-: version "0.86" ;
+: version "0.87" ;

vm/factor.c

@@ -104,7 +104,5 @@ int main(int argc, char** argv)
 
 	run_toplevel();
 
-	critical_error("run_toplevel() returned due to empty callstack",0);
-
 	return 0;
 }

vm/run.c

@@ -69,7 +69,12 @@ void interpreter_loop(void)
 		if(callframe_scan == callframe_end)
 		{
 			if(cs_bot - cs == CELLS)
-				return;
+			{
+				if(stack_chain->next)
+					return;
+
+				general_error(ERROR_CS_UNDERFLOW,F,F,false);
+			}
 
 			callframe_end = get(cs);
 			callframe_scan = get(cs - CELLS);