Model validation, initial cut

Fixes #11.

types/validation-error.go

@@ -0,0 +1,17 @@
+package types
+
+import "encoding/json"
+
+type ValidationError map[string][]string
+
+func (v ValidationError) Error() string {
+	errs, err := json.Marshal(struct {
+		ValidationError `json:"errors"`
+	}{v})
+
+	if err != nil {
+		return err.Error()
+	}
+
+	return string(errs)
+}