patch 8.0.1245: when WaitFor() has a wrong expression it just waits a second

Problem:    When WaitFor() has a wrong expression it just waits a second,
            which goes unnoticed. (James McCoy)
Solution:   When WaitFor() times out throw an exception.  Fix places where the
            expression was wrong.
diff --git a/src/testdir/shared.vim b/src/testdir/shared.vim
index 8db388a..eb43cf4 100644
--- a/src/testdir/shared.vim
+++ b/src/testdir/shared.vim
@@ -139,7 +139,7 @@
     endif
     sleep 10m
   endfor
-  return timeout
+  throw 'WaitFor() timed out after ' . timeout . ' msec'
 endfunc
 
 " Wait for up to a given milliseconds.