Error about provided team being invalid could be more helpful

[carols10cents]

Related to #343 but slightly different.

Over here: rust-lang/rfcs#3946 (comment)

I said:

@rfcbot fcp merge crates.io

and rfc bot responded with:

Error encountered:
Provided team crates.io is invalid

I wish it had responded with something like:

Error encountered:
Provided team crates.io is invalid

Valid team names

Please specify one or more of these team names, separated by commas:

• compiler
• crates-io
• [etc]

so that I could know that "crates.io" needed to be "crates-io" without having to go look at the team repo.