New behaviour in case of low skill during building

Insufficient skill after hitting the next improvement stage is now
handled by leaving the for-loop and skipping to after-build-processing.

src/kernel/build.c

@@ -522,7 +522,9 @@ int build(unit * u, const construction * ctype, int completed, int want)
 
         if (basesk < type->minskill) {
             if (made == 0)
-                return ELOWSKILL;       /* not good enough to go on */
+                return ELOWSKILL;
+            else
+                break;              /* not good enough to go on */
         }
 
         /* n = maximum buildable size */