patch 8.2.2905: no error when defaults.vim cannot be loaded

Problem:    No error when defaults.vim cannot be loaded.
Solution:   Add an error message. (Christian Brabandt, closes #8248)
diff --git a/src/main.c b/src/main.c
index 277e317..db8202e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -3128,7 +3128,11 @@
     if (parmp->use_vimrc != NULL)
     {
 	if (STRCMP(parmp->use_vimrc, "DEFAULTS") == 0)
-	    do_source((char_u *)VIM_DEFAULTS_FILE, FALSE, DOSO_NONE, NULL);
+	{
+	    if (do_source((char_u *)VIM_DEFAULTS_FILE, FALSE, DOSO_NONE, NULL)
+									 != OK)
+		emsg(e_failed_to_source_defaults);
+	}
 	else if (STRCMP(parmp->use_vimrc, "NONE") == 0
 				     || STRCMP(parmp->use_vimrc, "NORC") == 0)
 	{
@@ -3200,7 +3204,9 @@
 		&& !has_dash_c_arg)
 	    {
 		// When no .vimrc file was found: source defaults.vim.
-		do_source((char_u *)VIM_DEFAULTS_FILE, FALSE, DOSO_NONE, NULL);
+		if (do_source((char_u *)VIM_DEFAULTS_FILE, FALSE, DOSO_NONE,
+								 NULL) == FAIL)
+		    emsg(e_failed_to_source_defaults);
 	    }
 	}