display server errors when creating pullrequest fails

Closes mislav#114

lib/hub/commands.rb

@@ -970,7 +970,13 @@ def self.===(exception)
 
     def display_http_exception(action, response)
       $stderr.puts "Error #{action}: #{response.message} (HTTP #{response.code})"
-      warn "Check your token configuration (`git config github.token`)" if response.code.to_i == 401
+      case response.code.to_i
+      when 401 then warn "Check your token configuration (`git config github.token`)"
+      when 422
+        if response.content_type =~ /\bjson\b/ and data = JSON.parse(response.body) and data["error"]
+          $stderr.puts data["error"]
+        end
+      end
     end
 
   end

test/hub_test.rb

@@ -882,6 +882,17 @@ def test_pullrequest_existing_issue_url
     assert_output expected, "pull-request https://github.com/mojombo/hub/issues/92#comment_4"
   end
 
+  def test_pullrequest_fails
+    stub_request(:post, "https://#{auth}github.com/api/v2/json/pulls/defunkt/hub").
+      to_return(:status => [422, "Unprocessable Entity"],
+                :headers => {"Content-type" => "application/json"},
+                :body => %({"error":["oh no!", "it failed."]}))
+
+    expected = "Error creating pull request: Unprocessable Entity (HTTP 422)\n"
+    expected << "oh no!\nit failed.\n"
+    assert_output expected, "pull-request hereyougo -b feature -f"
+  end
+
   def test_checkout_no_changes
     assert_forwarded "checkout master"
   end