patch 8.1.1524: tests are silently skipped

Problem:    Tests are silently skipped.
Solution:   Throw an exception for skipped tests in more places.
diff --git a/src/testdir/test_reltime.vim b/src/testdir/test_reltime.vim
index adabf16..1876539 100644
--- a/src/testdir/test_reltime.vim
+++ b/src/testdir/test_reltime.vim
@@ -1,7 +1,10 @@
 " Tests for reltime()
 
-if !has('reltime') || !has('float')
-  finish
+if !has('reltime')
+  throw 'Skipped, reltime feature missing'
+endif
+if !has('float')
+  throw 'Skipped, float feature missing'
 endif
 
 func Test_reltime()