Better check that pull request creation errors have details

common/lib/dependabot/pull_request_creator/github.rb

@@ -289,7 +289,7 @@ def create_pull_request
           headers: custom_headers || {}
         )
       rescue Octokit::UnprocessableEntity => e
-        return handle_pr_creation_error(e) if e.message.include?("field:")
+        return handle_pr_creation_error(e) if e.message.include? "Error summary"
 
         # Sometimes PR creation fails with no details (presumably because the
         # details are internal). It doesn't hurt to retry in these cases, in

common/spec/dependabot/pull_request_creator/github_spec.rb

@@ -416,6 +416,34 @@
           expect(WebMock).to_not have_requested(:post, "#{repo_api_url}/pulls")
         end
 
+        context "but isn't initially returned (a race)" do
+          before do
+            url = "#{repo_api_url}/pulls?head=gocardless:#{branch_name}"\
+                  "&state=all"
+            stub_request(:get, url).
+              to_return(status: 200, body: "[]", headers: json_header)
+            stub_request(
+              :patch,
+              "#{repo_api_url}/git/refs/heads/#{branch_name}"
+            ).to_return(
+              status: 200,
+              body: fixture("github", "update_ref.json"),
+              headers: json_header
+            )
+            stub_request(:post, "#{repo_api_url}/pulls").
+              to_return(
+                status: 422,
+                body: fixture("github", "pull_request_already_exists.json"),
+                headers: json_header
+              )
+          end
+
+          it "returns nil" do
+            expect(creator.create).to be_nil
+            expect(WebMock).to have_requested(:post, "#{repo_api_url}/pulls")
+          end
+        end
+
         context "but is merged" do
           before do
             url = "#{repo_api_url}/pulls?head=gocardless:#{branch_name}"\

common/spec/fixtures/github/pull_request_already_exists.json

@@ -0,0 +1,11 @@
+{
+    "message": "Validation Failed",
+    "errors": [
+        {
+            "resource": "PullRequest",
+            "code": "custom",
+            "message": "A pull request already exists for base branch 'develop' and head branch 'dependabot/maven/com.fasterxml.jackson.dataformat-jackson-dataformat-xml-2.9.7"
+        }
+    ],
+    "documentation_url": "https://developer.github.com/v3/pulls/#update-a-pull-request"
+}