patch 8.2.1593: tests do not check the error number properly

Problem:    Tests do not check the error number properly.0
Solution:   Add a colon after the error number. (closes #6869)
diff --git a/src/testdir/test_gui.vim b/src/testdir/test_gui.vim
index c01a290..64537ac 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -525,7 +525,7 @@
         set guifontset=-*-notexist-*
         call assert_report("'set guifontset=-*-notexist-*' should have failed")
       catch
-        call assert_exception('E598')
+        call assert_exception('E598:')
       endtry
       " Set it to an invalid value brutally for preparation.
       let &guifontset = '-*-notexist-*'