Make error message clearer when payment fails due to not enough funds

[crape-swans]

Overview

When a payment fails because there aren't enough funds, the error message isn't very clear, and users may not understand what went wrong or how to fix it.
The current error message doesn't directly say that the failure is because of insufficient funds. This can confuse users, as they might not know if the issue is with the payment details, the network, or their balance.

Expected Behavior

The error message should clearly explain that the payment failed because there aren’t enough funds and suggest that the user check their balance or try a different payment method.

Proposed Solution

Update the error message to say that there aren't enough funds to complete the payment and provide a simple suggestion on how to fix it.

[rahulyadav-57]

@crape-swans In cases where a payment fails due to insufficient funds, the error code insufficient_funds is returned. If this error is received, you may display an appropriate, user-friendly custom message to the end user.

{
  x402Version: 2,
  error: 'insufficient_funds',
  resource: {
    url: 'http://localhost:4021/protected',
    description: 'Payment of $4 required to access \"Protected content\"',
    mimeType: 'application/json'
  },
  accepts: [
    {
      scheme: 'exact',
      network: 'eip155:84532',
      amount: '4000000',
      asset: '[redacted]',
      payTo: '[redacted]',
      maxTimeoutSeconds: 300,
      extra: {
        "name": "USDC",
        "version": "2"
      }
    }
  ]
}