check access to github

If someone has a BLAB_DATA repository set to use ssh and their key is no longer valid, they'll get a very cryptic error: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