A lots of bots on Chromium waterfall ran into this error.
One example: https://uberchromegw.corp.google.com/i/chromium.gpu/builders/Linux%20Release%20%28NVIDIA%29/builds/78832

9294 2016-05-25 20:32:11.486 E: Unable to open given url, https://chromium-swarm.appspot.com/_ah/api/swarming/v1/tasks/new, after 30 attempts.
503 Server Error: Service Unavailable for url: https://chromium-swarm.appspot.com/_ah/api/swarming/v1/tasks/new
----------
Alternate-protocol: 443:quic
X-xss-protection: 1; mode=block
X-content-type-options: nosniff
Content-encoding: gzip
Transfer-encoding: chunked
Expires: Wed, 25 May 2016 20:32:11 GMT
Server: GSE
Cache-control: private, max-age=0
Date: Wed, 25 May 2016 20:32:11 GMT
X-frame-options: SAMEORIGIN
Alt-svc: quic=":443"; ma=2592000; v="34,33,32,31,30,29,28,27,26,25"
Content-type: application/json; charset=UTF-8

{
 "error": {
  "errors": [
   {
    "domain": "global",
    "reason": "backendError",
    "message": "Internal Server Error"
   }
  ],
  "code": 503,
  "message": "Internal Server Error"
 }
}