Fix for #1350, nonexisting build path in parent section causes extending section to fail

Signed-off-by: Timothy Van Heest <timothy.vanheest@gmail.com>

tests/fixtures/extends/nonexistent-path-base.yml

@@ -0,0 +1,6 @@
+dnebase:
+  build: nonexistent.path
+  command: /bin/true
+  environment:
+    - FOO=1
+    - BAR=1

tests/fixtures/extends/nonexistent-path-child.yml

@@ -0,0 +1,8 @@
+dnechild:
+  extends:
+    file: nonexistent-path-base.yml
+    service: dnebase
+  image: busybox
+  command: /bin/true
+  environment:
+    - BAR=2