Don't open second MR for gradle (Closes: #914)

Traceback (most recent call last):
  File "/builds/fdroid/fdroidserver/./tests/gradle-release-checksums.py", line 130, in <module>
    mr = project.mergerequests.create({
  File "/usr/lib/python3/dist-packages/gitlab/exceptions.py", line 281, in wrapped_f
    raise error(e.error_message, e.response_code, e.response_body) from e
gitlab.exceptions.GitlabCreateError: 409: ['Another open merge request already exists for this source branch: !1064']

tests/gradle-release-checksums.py

@@ -127,13 +127,16 @@ if git_repo.is_dirty() and ('gradlew-fdroid' in modified or 'makebuildserver' in
         '\n\n<p><small>generated by <a href="%s/-/jobs/%s">GitLab CI Job #%s</a></small></p>'
         % (os.getenv('CI_PROJECT_URL'), os.getenv('CI_JOB_ID'), os.getenv('CI_JOB_ID'))
     )
-    mr = project.mergerequests.create({
-        'source_branch': branch.name,
-        'target_project_id': 36527,  # fdroid/fdroidserver
-        'target_branch': 'master',
-        'title': 'update to gradle v' + version,
-        'description': description,
-        'labels': ['fdroid-bot', 'gradle'],
-        'remove_source_branch': True,
-    })
-    mr.save()
+    try:
+        mr = project.mergerequests.create({
+            'source_branch': branch.name,
+            'target_project_id': 36527,  # fdroid/fdroidserver
+            'target_branch': 'master',
+            'title': 'update to gradle v' + version,
+            'description': description,
+            'labels': ['fdroid-bot', 'gradle'],
+            'remove_source_branch': True,
+        })
+        mr.save()
+    except gitlab.exceptions.GitlabCreateError as e:
+        print(e.error_message)