Fix error message when Cargo.toml misses a [package] section

[mdevlamynck]

Fixes #3388.

[rust-highfive]

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[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 4cd544c has been approved by alexcrichton

[bors]

⌛ Testing commit 4cd544c with merge 0826f27...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 0826f27 to master...