Vscode-pull-request-github: Fetching pull requests fails with "Not Found"

[RMacfarlane]

@ramya-rao-a has a reliable repro, I haven't been able to repro on my machine

Clone Microsoft/vscode-go
When expanding any of the category nodes in the tree that isn't just "Local Pull Requests", the octokit/rest library throws

GitHubRepository> Fetching all pull requests failed: {"message":"Not Found","documentation_url":"https://developer.github.com/v3/pulls/#list-pull-requests"}

[RMacfarlane]

Ramya's git config:

[core]
    repositoryformatversion = 0
    filemode = false
    bare = false
    logallrefupdates = true
    symlinks = false
    ignorecase = true
[remote "origin"]
    url = https://github.com/Microsoft/vscode-go
    fetch = +refs/heads/*:refs/remotes/origin/*
[remote "uudashr"]
    url = https://github.com/uudashr/vscode-go
    fetch = +refs/heads/*:refs/remotes/uudashr/*
[remote "ramyar"]
    url = https://github.com/ramya-rao-a/vscode-go
    fetch = +refs/heads/*:refs/remotes/ramyar/*
[remote "juicemia"]
    url = https://github.com/juicemia/vscode-go
    fetch = +refs/heads/*:refs/remotes/juicemia/*
[remote "zhouhaibing089"]
    url = https://github.com/zhouhaibing089/vscode-go
    fetch = +refs/heads/*:refs/remotes/zhouhaibing089/*
[branch "dlv-test-pkg"]
    remote = zhouhaibing089
    merge = refs/heads/dlv-test-pkg
[remote "apiarian"]
    url = https://github.com/apiarian/vscode-go
    fetch = +refs/heads/*:refs/remotes/apiarian/*
[branch "godoc-view"]
    remote = apiarian
    merge = refs/heads/godoc-view
[remote "m90"]
    url = https://github.com/m90/vscode-go
    fetch = +refs/heads/*:refs/remotes/m90/*
[branch "playground"]
    remote = m90
    merge = refs/heads/playground
[remote "ndrewnee"]
    url = https://github.com/ndrewnee/vscode-go
    fetch = +refs/heads/*:refs/remotes/ndrewnee/*
[remote "buyology"]
    url = https://github.com/buyology/vscode-go
    fetch = +refs/heads/*:refs/remotes/buyology/*
[branch "buildtags"]
    remote = origin
    merge = refs/heads/buildtags
[branch "master"]
    remote = origin
    merge = refs/heads/master
[remote "hanleym"]
    url = https://github.com/hanleym/vscode-go
    fetch = +refs/heads/*:refs/remotes/hanleym/*
[branch "expanderr"]
    remote = hanleym
    merge = refs/heads/master
[remote "cheesedosa"]
    url = https://github.com/cheesedosa/vscode-go
    fetch = +refs/heads/*:refs/remotes/cheesedosa/*
[branch "fix_1456"]
    remote = cheesedosa
    merge = refs/heads/fix_1456
[branch "goroot-symbols"]
    remote = m90
    merge = refs/heads/goroot-symbols
[remote "joncalhoun"]
    url = https://github.com/joncalhoun/vscode-go
    fetch = +refs/heads/*:refs/remotes/joncalhoun/*
[branch "new"]
    remote = joncalhoun
    merge = refs/heads/master
[remote "lggomez"]
    url = https://github.com/lggomez/vscode-go
    fetch = +refs/heads/*:refs/remotes/lggomez/*
[branch "delve_v2"]
    remote = lggomez
    merge = refs/heads/delve_v2
[remote "mbenkmann"]
    url = https://github.com/mbenkmann/vscode-go
    fetch = +refs/heads/*:refs/remotes/mbenkmann/*
[branch "gofromat"]
    remote = mbenkmann
    merge = refs/heads/master
[branch "gopkgs-workdir"]
    remote = uudashr
    merge = refs/heads/gopkgs-workdir
[remote "ikedam"]
    url = https://github.com/ikedam/vscode-go
    fetch = +refs/heads/*:refs/remotes/ikedam/*
[remote "tylerb"]
    url = https://github.com/tylerb/vscode-go
    fetch = +refs/heads/*:refs/remotes/tylerb/*
[branch "cancel-test"]
    remote = tylerb
    merge = refs/heads/master
[branch "feature/toolCommands"]
    remote = ikedam
    merge = refs/heads/feature/toolCommands
[remote "mhr3"]
    url = https://github.com/mhr3/vscode-go
    fetch = +refs/heads/*:refs/remotes/mhr3/*
[branch "testify-support"]
    remote = mhr3
    merge = refs/heads/testify-support
[remote "xmattstrongx"]
    url = https://github.com/xmattstrongx/vscode-go
    fetch = +refs/heads/*:refs/remotes/xmattstrongx/*
[branch "strlen"]
    remote = xmattstrongx
    merge = refs/heads/master
[remote "freman"]
    url = https://github.com/freman/vscode-go
    fetch = +refs/heads/*:refs/remotes/freman/*
[branch "feat-1733"]
    remote = freman
    merge = refs/heads/feat-1733

Using this, the error reliably repros. Some remotes point to forks that no longer exist, for example https://github.com/buyology/vscode-go

[ramya-rao-a]

Now everybody knows all the remotes I ever used...
Look at all those contributors!