[Feature request]: Print names of all examples when --examples is missing a value.

[richard-uk1]

If you do cargo run --example, you will get an error saying that a value was expected, but none was supplied.

It would be great if you also got a list of the available examples.

I'm not sure if clap currently supports this functionality, either

1. Custom error path when an option with name example is missing a value.
2. Ability to specify that a value is optional, and then manually call the clap error handler with the extra info.

[richard-uk1]

I thought this may just not be possible because of the ambiguous situation cargo run --example --test, but I see that clap already rejects this, requiring it to be written as cargo run --example=--test.

[ehuss]

Thanks for the report. This is a duplicate of #2548, so closing in favor of that.

[richard-uk1]

Yep, thought I'd checked for dupes sorry!