Add ProfessorAlreadyExistsError (#13)

* add ProfessorAlreadyExistsError and rescue it in #find_or_create_professor

* refine rescue

* gst

* move rescue in #create_professor

lib/edusign/client.rb

@@ -236,6 +236,9 @@ def create_professor(first_name:, last_name:, email:)
         dontSendCredentials: true
       }
       api :post, "/professor", payload.to_json
+    rescue Response::ProfessorAlreadyExistsError
+      # NOTE(Eschults): try again finding the professor
+      find_or_create_professor(first_name: first_name, last_name: last_name, email: email)
     end
 
     def find_or_create_professor(first_name:, last_name:, email:)
@@ -279,6 +282,7 @@ def api(http_method, path, body = {}, opts = {})
       raise GatewayTimeoutError, request.message if request.code == 504
 
       response = Response.new(JSON.parse(request.body, symbolize_names: true))
+      raise Response::ProfessorAlreadyExistsError, response.message if response.message == Response::ProfessorAlreadyExistsError::PROFESSOR_ALREADY_EXISTS_MESSAGE
       raise Response::Error, response.message if response.error?
 
       response
@@ -296,6 +300,10 @@ class Error < StandardError
         CANNOT_GET_PROFESSOR_BY_EMAIL_ERROR_MESSAGE = "Error - cannot get professor by email".freeze
       end
 
+      class ProfessorAlreadyExistsError
+        PROFESSOR_ALREADY_EXISTS_MESSAGE = "Professor already exists in this school".freeze
+      end
+
       attr_reader :body
 
       def initialize(body)