Django:如何覆盖默认错误消息？

from django import forms

class MyForm(forms.Form):
    my_field = forms.CharField()

    def clean_my_field(self):
        data = self.cleaned_data['my_field']
        if len(data) < 5:
            raise forms.ValidationError("自定义错误消息：字段长度不能少于5个字符")
        return data

from django.db import models

class MyModel(models.Model):
    my_field = models.CharField(max_length=100, error_messages={
        'blank': "自定义错误消息：该字段不能为空",
        'max_length': "自定义错误消息：字段长度不能超过100个字符",
    })

from django.shortcuts import render
from .forms import MyForm

def my_view(request):
    if request.method == 'POST':
        form = MyForm(request.POST)
        if form.is_valid():
            # 处理有效的表单数据
            pass
    else:
        form = MyForm()

    # 添加自定义错误消息
    form.add_error('my_field', "自定义错误消息：发生了一个错误")

    return render(request, 'my_template.html', {'form': form})