Limit number of pages a document can have on upload.

[duckduckgrayduck]

It raises a PageCountError which gets returned in the API. Here is me trying to upload a 2001 page document that lives on production on staging:
https://api.staging.documentcloud.org/api/documents/20014762/errors/

Closes part 1 of #479

[eyeseast]

This shows up in Sentry: https://muckrock.sentry.io/issues/7748986224/?alert_rule_id=927646&alert_timestamp=1790099553878&alert_type=email&environment=production&notification_uuid=6287b185-c3e7-4aa7-8509-785c6fd03e41&project=1844864&referrer=alert_email

I think that's fine, actually. We'll get some insight into how often people are hitting this limit and can decide what to do with that.

[eyeseast]

I was able to upload a document on staging. The upload succeeds but processing fails, and we end up in this weird state: https://www.staging.documentcloud.org/documents/20014763-south-carolina-hearing-office-alj-dispositions-01-01-10-to-02-21-18/

This was something already on staging, so here's the original file: https://documentcloud-staging-files.s3.amazonaws.com/documents/20010618/south-carolina-hearing-office-alj-dispositions-01-01-10-to-02-21-18.pdf

We may need a check on the frontend to catch this earlier, or a better way to indicate that processing failed. These are probably follow-on issues.

Here's how it looks in the API:

• Metadata: https://api.staging.documentcloud.org/api/documents/20014763/
• Errors: https://api.staging.documentcloud.org/api/documents/20014763/errors/

[duckduckgrayduck]

Yea, it raises PageCountError() intentionally, similar to PdfSizeError().
Looking at that code, the PdfSizeError doesn't set a message which is observable in errors like this one is, it returns blank "" so errors swallows it. Since I am here, I am going to add a message for this too, and then the front-end can use that info to display a helpful failure message