hush: document a bug about aborting on sourced file error when non-interactive

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-misc/source1.right b/shell/hush_test/hush-misc/source1.right
new file mode 100644
index 0000000..d425603
--- /dev/null
+++ b/shell/hush_test/hush-misc/source1.right
@@ -0,0 +1,5 @@
+hush: syntax error: unterminated ${name}
+line2
+Ok1:0
+hush: syntax error: unterminated '
+Ok2:1