Handle API errors when the token has issues (invalid or low credits)

[ignacio-chiazzo]

Describe the bug
When the API token has issues, the gem raises a Faraday error. This can happen for several reasons, such as an invalid API token or low credit, etc.

To Reproduce

require 'bundler/inline'

gemfile(true) do
  source 'https://rubygems.org'

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "anthropic", path: "../"
end

require 'anthropic'


# secret key
client = Anthropic::Client.new(access_token: "invalid-key")

binding.pry
response = client.messages(
  parameters: {
    model: "claude-3-sonnet-20240229", 
    system: "Respond only in Spanish.",
    messages: [
      {"role": "user", "content": "Hello, Claude!"}
    ],
    max_tokens: 1000
  }
)

Expected behavior
A defined error is raised.

actual behavior
Faraday raises an error:

from (pry):3:in `on_complete'
gems/faraday-2.12.0/lib/faraday/response/raise_error.rb:31:in `on_complete': the server responded with status 400 (Faraday::BadRequestError)"

I inspected the API response and got the following error:

" @status=400 @reason_phrase="Bad Request" @response_body={"type"=>"error", "error"=>{"type"=>"invalid_request_error", "message"=>"Your credit balance is too low to access the Anthropic API. Please go to Plans & Billing to upgrade or purchase credits."}}>>

If you're okay with API consumers catching the API errors (4XX), I will close this issue and update the README. Thanks!