Get substring from a string

I have this large response from an external API call, and I want to get only a specific message from the string.

Here is the response:

     "There was a problem sending the email through the SendGrid API.\n\nHere is the 
      response:\n\n\"{\\\"errors\\\":[{\\\"message\\\":\\\"The subject is required. You can get around this 
     requirement if you use a template with a subject defined or if every personalization has a subject defined.\\\"

I want to extract the message value from the above string. message will contain any value depending on the error. In this case it is this line:

                      "The subject is required. You can get around this 
     requirement if you use a template with a subject defined or if every personalization has a subject defined.\\\"

Is there any regex that can help?

Any help will be much appreciated
Thanks.

What email sending library you use in Elixir?