patch 8.1.1483: skipped tests are not properly listed

Problem:    Skipped tests are not properly listed.
Solution:   Throw a "Skipped" exception instead of using ":finish" or ":return".
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index cb86186..bfd39d5 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -655,7 +655,7 @@
 func Test_textprop_screenshot_various()
   " The Vim running in the terminal needs to use utf-8.
   if !CanRunVimInTerminal() || g:orig_encoding != 'utf-8'
-    return
+    throw 'Skipped: cannot make screendumps or not using utf-8'
   endif
   call writefile([
 	\ "call setline(1, ["
@@ -750,7 +750,7 @@
 " screenshot test with Visual block mode operations
 func Test_textprop_screenshot_visual()
   if !CanRunVimInTerminal()
-    return
+    throw 'Skipped: cannot make screendumps'
   endif
 
   " Delete two columns while text props are three chars wide.