Print warning when no fixture was found

Prints a warning to `stderr` when a fixture couldn't be found for a
resource ID as this is likely going to lead to problems.

generator.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 
 	"github.com/stripe/stripe-mock/spec"
+	"github.com/stripe/stripe-mock/util"
 )
 
 var errExpansionNotSupported = fmt.Errorf("Expansion not supported")
@@ -111,6 +112,7 @@ func (g *DataGenerator) generateResource(schema *spec.JSONSchema) (interface{},
 
 	fixture, ok := g.fixtures.Resources[spec.ResourceID(schema.XResourceID)]
 	if !ok {
+		util.Warningf("No fixture for: %s", spec.ResourceID(schema.XResourceID))
 		return map[string]interface{}{}, nil
 	}
 

util/util.go

@@ -0,0 +1,11 @@
+package util
+
+import (
+	"fmt"
+	"os"
+)
+
+func Warningf(format string, a ...interface{}) {
+	format = "WARNING: " + format + "\n"
+	fmt.Fprintf(os.Stderr, format, a...)
+}

util/util_test.go

@@ -0,0 +1 @@
+package util