commands: fix mismatch in argument error reporting

[bboreham]

It was parsing args[0] but reporting an error on args[1], which might panic and obscure the error.

Also do the initial parsing earlier, to save effort reading the core if we can't proceed.

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[gopherbot]

Message from Keith Randall:

Patch Set 1: Auto-Submit+1 Code-Review+2 Commit-Queue+1

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Patch Set 1:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-02-29T23:00:57Z","revision":"46c5795f0e5d1a9df113c6772f6a3aeec26502bd"}

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Patch Set 1: Code-Review+1

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Patch Set 1: -Commit-Queue

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Patch Set 1: LUCI-TryBot-Result+1

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Message from Bryan Boreham:

Patch Set 1:

(2 comments)

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Patch Set 2:

(1 comment)

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