gracefully handle GitHub server issues

This is probably impossible to reproduce, but I'll leave it here just in case. I feel like GitHub has had some server issues today. One manifestation of that was the following error from `gs stack submit` on an existing stack of PRs, where I had added a commit to the bottom of the stack:

```
username repo % gs ss
INF Updated #216: https://github.com/repo/pull/216
INF Updated #217: https://github.com/repo/pull/217
FTL gs: submit he/migrate-docs-to-makie: edit CR #218: edit pull request: non-200 OK status code: 504 Gateway Timeout body: "\r\n\r\n\r\n \r\n Unicorn! · GitHub\r\n \r\n body {\r\n background-color: #f1f1f1;\r\n margin: 0;\r\n font-family: \"Helvetica Neue\", Helvetica, Arial, sans-serif;\r\n }\r\n\r\n .container { margin: 50px auto 40px auto; width: 600px; text-align: center; }\r\n\r\n a { color: #4183c4; text-decoration: none; }\r\n a:hover { text-decoration: underline; }\r\n\r\n h1 { letter-spacing: -1px; line-height: 60px; font-size: 60px; font-weight: 100; margin: 0px; text-shadow: 0 1px 0 #fff; }\r\n p { color: rgba(0, 0, 0, 0.5); margin: 10px 0 10px; font-size: 18px; font-weight: 200; line-height: 1.6em;}\r\n\r\n ul { list-style: none; margin: 25px 0; padding: 0; }\r\n li { display: table-cell; font-weight: bold; width: 1%; }\r\n\r\n .logo { display: inline-block; margin-top: 35px; }\r\n .logo-img-2x { display: none; }\r\n @media\r\n only screen and (-webkit-min-device-pixel-ratio: 2),\r\n only screen and ( min--moz-device-pixel-ratio: 2),\r\n only screen and ( -o-min-device-pixel-ratio: 2/1),\r\n only screen and ( min-device-pixel-ratio: 2),\r\n only screen and ( min-resolution: 192dpi),\r\n only screen and ( min-resolution: 2dppx) {\r\n .logo-img-1x { display: none; }\r\n .logo-img-2x { display: inline-block; }\r\n }\r\n\r\n #suggestions {\r\n margin-top: 35px;\r\n color: #ccc;\r\n }\r\n #suggestions a {\r\n color: #666666;\r\n font-weight: 200;\r\n font-size: 14px;\r\n margin: 0 10px;\r\n }\r\n\r\n \r\n \r\n \r\n\r\n