Output when multicall applet isn't found is unhelpful and ugly

[fishface60]

Affected Version of clap

• 3.0.0-beta.4

Expected Behavior Summary

When I run a multicall program from a link that does not match an applet, I expect an error message explaining that the applet was not found and which applets exist.

Actual Behavior Summary

Error message just lists that the argument wasn't expected.

Steps to Reproduce the issue

$ cargo build --examples
$ ln target/debug/examples/24_multicall_busybox asdf
$ ./asdf 
error: Found argument 'asdf' which wasn't expected, or isn't valid in this context

USAGE:
     [SUBCOMMAND]

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