Vendor godep v79 and use it

vendor/github.com/tools/godep/errors.go

@@ -0,0 +1,18 @@
+package main
+
+import "errors"
+
+var (
+	errorLoadingDeps         = errors.New("error loading dependencies")
+	errorLoadingPackages     = errors.New("error loading packages")
+	errorCopyingSourceCode   = errors.New("error copying source code")
+	errorNoPackagesUpdatable = errors.New("no packages can be updated")
+)
+
+type errPackageNotFound struct {
+	path string
+}
+
+func (e errPackageNotFound) Error() string {
+	return "Package (" + e.path + ") not found"
+}