Add vendor (steam)

vendor/github.com/Philipp15b/go-steam/tradeoffer/error.go

@@ -0,0 +1,19 @@
+package tradeoffer
+
+import (
+	"fmt"
+)
+
+// SteamError can be returned by Create, Accept, Decline and Cancel methods.
+// It means we got response from steam, but it was in unknown format
+// or request was declined.
+type SteamError struct {
+	msg string
+}
+
+func (e *SteamError) Error() string {
+	return e.msg
+}
+func newSteamErrorf(format string, a ...interface{}) *SteamError {
+	return &SteamError{fmt.Sprintf(format, a...)}
+}