Update OOB errors

[lionel-]

• Use standard subscript error header in OOB errors.
• Use actual vs expected phrasing.
• Merge OOB and non-consecutive OOB errors. Instead of a specific error, just mention in an info bullet that consecutive extension is allowed.
• When multiple locations are problematic, mention them in their own cross bullet.

[lionel-]

This is a case where chained errors might fit better because there are multiple bullets "can't / must" bullets describing a problem at different levels.

Same for the "must" cross bullet above.

[hadley]

I think this is an improvement, but it feels like we can still do even better, so I'd like to push this off until the next release so we can spend more time on it, and do it once.

[hadley]

Do we really need "Can't subset elements" in these three errors?

[lionel-]

The thing is that this is this header that carries the arg information, if any. That will be the case when called from select() and tibble's subset methods.

[hadley]

Could this be one line like Can't find location 10; input is length 2?

[lionel-]

This is slightly problematic because these errors are used for subsetting across tibble rows and cols. So it's hard to find generic formulations that work well for all cases.

[lionel-]

We could say "input has 2 elements" or "input has 3 rows". This is how we currently phrase it in an additional info bullet, but I guess you're saying this information should be closer?

Another difficult aspect is how to handle the case where multiple locations are not found without creating a too long line. To address this, I try to consistently mention multiple locations in their own bullet in this PR. But then it's hard to keep the length info close.