Refuse a repo name that is not an owner/repo String in repo_id_by_name

[Thayorns]

repo_id_by_name checked its argument only for nil. A name without a slash, or an empty one, reached Octokit and came back as a raw Octokit::InvalidRepository, and an Integer went straight to GET /repositories/:id, as #1228 shows. Its sibling repo_name_by_id already refuses a value of the wrong type with Fbe::Error.

Now repo_id_by_name raises Fbe::Error, naming the value, unless it is a String with exactly one slash and something on both sides of it. The check runs before any request.

The tests cover a random name without a slash, an empty name, an Integer, a name with two slashes, one without the owner and one without the repository, all on the fake client, and the slash-less name from the issue on the real Octokit client through WebMock.

Closes #1228

[Thayorns]

@yegor256 take a look please, happy to clarify anything about this change.