additional error handling; use path buffers; improve logic

demo/tmpl/ex-edge-r/interfaces.tera

@@ -0,0 +1,8 @@
+interfaces {
+{%- set interfaces = uplinks | default(value=[uplink]) %}
+{%- for interace in interfaces %}
+    {{ interace }} {
+        unit 0 family inet dhcp;
+    }
+{%- endfor %}
+}