bump-formula-pr: use GitHub error message info

This saves an API call, and is more accurate, because the repo API
doesn't actually say whether forking is enabled, but this error message
does.

To do this, the original GitHub error message had to be accessible on
the GitHub exceptions.

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -366,15 +366,12 @@ module Homebrew
             remote_url = response.fetch("clone_url")
           end
           username = response.fetch("owner").fetch("login")
-        rescue *GitHub.api_errors
+        rescue GitHub::AuthenticationFailedError => e
+          raise unless e.github_message =~ /forking is disabled/
           # If the repository is private, forking might be disabled.
           # Create branches in the repository itself instead.
           remote_url = Utils.popen_read("git remote get-url --push origin").chomp
           username = formula.tap.user
-          repo_name = "homebrew-#{formula.tap.repo}"
-          unless GitHub.repository(username, repo_name).fetch("private")
-            raise
-          end
         rescue *GitHub.api_errors => e
           formula.path.atomic_write(backup_file) unless args.dry_run?
           odie "Unable to fork: #{e.message}!"

Library/Homebrew/utils/github.rb

@@ -15,13 +15,22 @@ module GitHub
   PR_ENV_KEY = "HOMEBREW_NEW_FORMULA_PULL_REQUEST_URL".freeze
   PR_ENV = ENV[PR_ENV_KEY]
 
-  Error = Class.new(RuntimeError)
-  HTTPNotFoundError = Class.new(Error)
+  class Error < RuntimeError
+    attr_reader :github_message
+  end
+
+  class HTTPNotFoundError < Error
+    def initialize(github_message)
+      @github_message = github_message
+      super
+    end
+  end
 
   class RateLimitExceededError < Error
-    def initialize(reset, error)
+    def initialize(reset, github_message)
+      @github_message = github_message
       super <<~EOS
-        GitHub API Error: #{error}
+        GitHub API Error: #{github_message}
         Try again in #{pretty_ratelimit_reset(reset)}, or create a personal access token:
           #{ALL_SCOPES_URL}
         and then set the token as: export HOMEBREW_GITHUB_API_TOKEN="your_new_token"
@@ -34,8 +43,9 @@ module GitHub
   end
 
   class AuthenticationFailedError < Error
-    def initialize(error)
-      message = "GitHub #{error}\n"
+    def initialize(github_message)
+      @github_message = github_message
+      message = "GitHub #{github_message}\n"
       if ENV["HOMEBREW_GITHUB_API_TOKEN"]
         message << <<~EOS
           HOMEBREW_GITHUB_API_TOKEN may be invalid or expired; check:
@@ -193,6 +203,13 @@ module GitHub
   end
 
   def raise_api_error(output, errors, http_code, headers, scopes)
+    json = begin
+      JSON.parse(output)
+    rescue
+      nil
+    end
+    message = json&.[]("message") || "curl failed! #{errors}"
+
     meta = {}
     headers.lines.each do |l|
       key, _, value = l.delete(":").partition(" ")
@@ -204,25 +221,18 @@ module GitHub
 
     if meta.fetch("x-ratelimit-remaining", 1).to_i <= 0
       reset = meta.fetch("x-ratelimit-reset").to_i
-      error = JSON.parse(output)["message"]
-      raise RateLimitExceededError.new(reset, error)
+      raise RateLimitExceededError.new(reset, message)
     end
 
     GitHub.api_credentials_error_message(meta, scopes)
 
     case http_code
     when "401", "403"
-      raise AuthenticationFailedError, output
+      raise AuthenticationFailedError, message
     when "404"
-      raise HTTPNotFoundError, output
+      raise HTTPNotFoundError, message
     else
-      error = begin
-        JSON.parse(output)["message"]
-      rescue
-        nil
-      end
-      error ||= "curl failed! #{errors}"
-      raise Error, error
+      raise Error, message
     end
   end