put output errors behind a feature flag

We're going to start merging breaking functgionality behind feature
flags, to reduce the need for long-lived feature branches.

terraform/eval_output.go

@@ -68,6 +68,8 @@ func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
 
 	// handling the interpolation error
 	if err != nil {
+		switch {
+		case featureOutputErrors:
 			if n.ContinueOnErr {
 				log.Printf("[ERROR] Output interpolation %q failed: %s", n.Name, err)
 				// if we're continueing, make sure the output is included, and
@@ -78,8 +80,10 @@ func (n *EvalWriteOutput) Eval(ctx EvalContext) (interface{}, error) {
 				}
 				return nil, EvalEarlyExitError{}
 			}
-
 			return nil, err
+		default:
+			log.Printf("[WARN] Output interpolation %q failed: %s", n.Name, err)
+		}
 	}
 
 	// Get the value from the config

terraform/features.go

@@ -0,0 +1,9 @@
+package terraform
+
+import (
+	"os"
+)
+
+// This file holds feature flags for the next release
+
+var featureOutputErrors = os.Getenv("TF_OUTPUT_ERRORS") != ""