patch 8.2.0990: Using duplicate error number

Problem:    Using duplicate error number.
Solution:   Use an unused error number.  Add a test for it.
diff --git a/src/version.c b/src/version.c
index d69b560..57b092a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    990,
+/**/
     989,
 /**/
     988,