cargo login should try detecting errors

[sugar700]

When using cargo login, cargo asks for

please visit https://crates.io/me and paste the API Token below

While the input is trimmed, there is more that can be done, for instance removing cargo login from beginning of token (if an entire command is pasted for some reason) and detecting things that definitely aren't tokens (too short or too long to be them, having invalid token characters and so on).

[eminence]

Triage: removing cargo login from the pasted token was implemented in #7588, so I think this issue can be closed.

[ehuss]

Yea, I think this can be effectively closed. There isn't a specific set of valid characters (it is an opaque string), so I don't think there is much more that cargo can do (with a bearer token).