patch 9.0.2078: several problems with type aliases

Problem:  several problems with type aliases
Solution: Check for more error conditions, add tests,
          fix issues

Check for more error conditions and add additional tests

fixes  #13434
fixes  #13437
fixes  #13438
closes #13441

Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalvars.c b/src/evalvars.c
index 5c7bc12..fd3c937 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -1838,7 +1838,7 @@
 
     if (tv->v_type == VAR_TYPEALIAS)
     {
-	semsg(_(e_using_typealias_as_variable), tv->vval.v_typealias->ta_name);
+	semsg(_(e_using_typealias_as_value), tv->vval.v_typealias->ta_name);
 	return NULL;
     }
 
@@ -3979,7 +3979,7 @@
 
 	    if (di->di_tv.v_type == VAR_TYPEALIAS)
 	    {
-		semsg(_(e_using_typealias_as_variable),
+		semsg(_(e_cannot_modify_typealias),
 					    di->di_tv.vval.v_typealias->ta_name);
 		clear_tv(&di->di_tv);
 		goto failed;