check access to github

[kalenkovich]

If someone has a BLAB_DATA repository set to use ssh and their key is no longer valid, they'll get a very cryptic error:

The access error is also there but it doesn't cause an actual R error and instead causes a weird strsplit error.

How it should work:

• blabr looks for, e.g., ~/BLAB_DATA/cdi_spreadsheet.
  • If it doesn't exist, it should check whether an https request to github is possible.
    • If it is, suggest cloning.
    • If it isn't, say that it is BLab-only data.
  • If it does exist, try whatever ssh/https uri was set up for the "origin" remote.
    • If it works, continue.
    • If it doesn't, suggest changing to https.