patch 8.2.4821: crash when imported autoload script was deleted

Problem:    Crash when imported autoload script was deleted.
Solution:   Initialize local variable. (closes #10274)  Give a more meaningful
            error message.
diff --git a/src/vim9script.c b/src/vim9script.c
index 4f0190e..bb5f029 100644
--- a/src/vim9script.c
+++ b/src/vim9script.c
@@ -713,7 +713,10 @@
     if (script->sn_import_autoload && script->sn_state == SN_STATE_NOT_LOADED)
     {
 	if (do_source(script->sn_name, FALSE, DOSO_NONE, NULL) == FAIL)
+	{
+	    semsg(_(e_cant_open_file_str), script->sn_name);
 	    return -1;
+	}
     }
 
     // Find name in "script".