patch 8.2.4653: "import autoload" does not check the file name

Problem:    "import autoload" does not check the file name.
Solution:   Give an error if the file is not readable. (closes #10049)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index be358453..25f06e6 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -2116,7 +2116,7 @@
 	    // with UNIX it is possible to open a directory
 	    if (mch_isdir(ffname))
 	    {
-		semsg(_(e_src_is_directory), ffname);
+		semsg(_(e_str_is_directory), ffname);
 		return FAIL;
 	    }
 #endif