gn: Stop printing all help when the command is unknown
gn: Stop printing all help when the command is unknown

When a command is unrecognized (usually due to a typo), gn previously
printed all the top level help, which scrolled the error off the screen.
Now it just prints the list of known commands.

BUG=

Review-Url: https://codereview.chromium.org/1939603002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390999}

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