Make build fail when external_pipeline fails

middleman-core/features/builder.feature

@@ -43,6 +43,10 @@ Feature: Builder
     Given a built app at "empty-app"
     Then the exit status should be 1
 
+  Scenario: Build external_pipeline errors
+    Given a built app at "external-pipeline-error"
+    Then the exit status should be 1
+
   Scenario: Build alias (b)
     Given a fixture app "large-build-app"
     When I run `middleman b`

middleman-core/fixtures/external-pipeline-error/config.rb

@@ -0,0 +1,5 @@
+activate :external_pipeline,
+  name: :failing,
+  command: "mv does-not-exist tmp/file.js",
+  source: "tmp",
+  latency: 2

middleman-core/lib/middleman-core/extensions/external_pipeline.rb

@@ -34,6 +34,11 @@ class Middleman::Extensions::ExternalPipeline < ::Middleman::Extension
       end
     end
 
+    unless $?.success?
+      logger.error '== External: Command failed with non-zero exit status'
+      exit(1)
+    end
+
     @watcher.poll_once!
   rescue ::Errno::ENOENT => e
     logger.error "== External: Command failed with message: #{e.message}"