Creating or updating a MR returns more informative status codes.

Using the API library to create or update a merge request at the moment a 404 error is returned.
This is fine when the merge request in question does not exist, but does not provide good
information that for example a required attribute is missing.
A status code of 400 (Bad request) is returned when creating or updating a merge request
when either `source_branch` or `target_branch` is missing. A status code of 409 is returned
when `source_branch` and `target_branch` are the same. Tests are added for these cases.

lib/api/merge_requests.rb

@@ -60,6 +60,13 @@ module Gitlab
           merge_request.reload_code
           present merge_request, with: Entities::MergeRequest
         else
+          if merge_request.errors[:target_branch].any?
+            error!(merge_request.errors[:target_branch], 400)
+          elsif merge_request.errors[:source_branch].any?
+            error!(merge_request.errors[:source_branch], 400)
+          elsif merge_request.errors[:base].any?
+            error!(merge_request.errors[:base], 422)
+          end
           not_found!
         end
       end
@@ -88,6 +95,13 @@ module Gitlab
           merge_request.mark_as_unchecked
           present merge_request, with: Entities::MergeRequest
         else
+          if merge_request.errors[:target_branch].any?
+            error!(merge_request.errors[:target_branch], 400)
+          elsif merge_request.errors[:source_branch].any?
+            error!(merge_request.errors[:source_branch], 400)
+          elsif merge_request.errors[:base].any?
+            error!(merge_request.errors[:base], 422)
+          end
           not_found!
         end
       end

spec/requests/api/merge_requests_spec.rb

@@ -41,6 +41,24 @@ describe Gitlab::API do
       response.status.should == 201
       json_response['title'].should == 'Test merge_request'
     end
+
+    it "should return 422 when source_branch equals target_branch" do
+      post api("/projects/#{project.id}/merge_requests", user),
+        title: "Test merge_request", source_branch: "master", target_branch: "master", author: user
+      response.status.should == 422
+    end
+
+    it "should return 400 when source_branch is missing" do
+      post api("/projects/#{project.id}/merge_requests", user),
+        title: "Test merge_request", target_branch: "master", author: user
+      response.status.should == 400
+    end
+
+    it "should return 400 when target_branch is missing" do
+      post api("/projects/#{project.id}/merge_requests", user),
+        title: "Test merge_request", source_branch: "stable", author: user
+      response.status.should == 400
+    end
   end
 
   describe "PUT /projects/:id/merge_request/:merge_request_id" do
@@ -49,6 +67,18 @@ describe Gitlab::API do
       response.status.should == 200
       json_response['title'].should == 'New title'
     end
+
+    it "should return 422 when source_branch and target_branch are renamed the same" do
+      put api("/projects/#{project.id}/merge_request/#{merge_request.id}", user),
+        source_branch: "master", target_branch: "master"
+      response.status.should == 422
+    end
+
+    it "should return merge_request with renamed target_branch" do
+      put api("/projects/#{project.id}/merge_request/#{merge_request.id}", user), target_branch: "test"
+      response.status.should == 200
+      json_response['target_branch'].should == 'test'
+    end
   end
 
   describe "POST /projects/:id/merge_request/:merge_request_id/comments" do