Add error explanation for E0013

[robinst]

Part of #24407.

[rust-highfive]

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[nrc]

nit s/can not/cannot

[nrc]

Thanks for the PR! r=me with the two nits fixed.

[robinst]

Fixed the nits and amended the commit.

[alexcrichton]

@bors: r=nrc 6cdb57d

[Manishearth]

cc @michaelsproul

[michaelsproul]

@Manishearth: I think this plays nicely with the other approved PR, #24894. Cherry-picking them both didn't require any manual intervention, so I think this means Homu will merge them OK?

The PR for E0010 does clash with E0013 however, so it'll either have to be applied after a rebase or as part of a rollup.

[bors]

⌛ Testing commit 6cdb57d with merge 26c7635...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt