Improved error message for private *dependency* errors

[bestdan]

BLUF: Could we improve the error message for installing dependencies to:

1. note when the 404 error comes from a dependency, not the main repo
2. suggest the user switches to the Sys.setenv("GITHUB_PAT" = ###) method for multiple private installs
3. potentially pass the auth_token down to the private dependency repo call

---

Imagine your installing a package using remotes::install_github("foo", auth_token = "###"), and you get this error message:

Downloading GitHub repo foo
Error: HTTP error 404.
  Not Found

  Rate limit remaining: 50/60
  Rate limit reset at: 2019-02-14 15:39:37 UTC

You might spend a bit of time confused if:

1. the foo repo is not private
2. foo is private, but you supplied a valid auth token.

If foo has dependency bar, then it's actually bar which is 404'ing.

If a repo/package has a dependency which is a private repo, and you supply a github token as an argument to the install_github(auth_token), the auth_token is not passed to installing the dependencies. Traceback below.

Traceback()

traceback()
16: stop(github_error(res))
15: github_commit(username = remote$username, repo = remote$repo, 
        host = remote$host, ref = remote$ref, pat = remote$auth_token %||% 
            github_pat(), use_curl = use_curl)
14: remote_sha.github_remote(x)
13: remote_sha(x)
12: FUN(X[[i]], ...)
11: vapply(remote, function(x) remote_sha(x), character(1))
10: package_deps(deps, repos = repos, type = type)
9: nrow(cran_deps)
8: combine_deps(package_deps(deps, repos = repos, type = type), 
       remote_deps(pkg, ...))
7: dev_package_deps(pkgdir, repos = repos, dependencies = dependencies, 
       type = type, ...)
6: install_deps(pkgdir, dependencies = dependencies, quiet = quiet, 
       build = build, build_opts = build_opts, upgrade = upgrade, 
       repos = repos, type = type, ...)
5: install(source, dependencies = dependencies, upgrade = upgrade, 
       force = force, quiet = quiet, build = build, build_opts = build_opts, 
       repos = repos, type = type, ...)
4: FUN(X[[i]], ...)
3: vapply(remotes, install_remote, ..., FUN.VALUE = character(1))
2: install_remotes(remotes, auth_token = auth_token, host = host, 
       dependencies = dependencies, upgrade = upgrade, force = force, 
       quiet = quiet, build = build, build_opts = build_opts, repos = repos, 
       type = type, ...)
1: remotes::install_github("foo",  auth_token = token2, force = TRUE)

[bestdan]

This existing PR might solve the issue: https://github.com/r-lib/remotes/pull/266/files

BUT it looks like it currently only applies to gitlab, not github.