More explicit error message when corpus_id is not set in environment

To avoid the ugly 500 errors that env get when they try to use API endpoints with corpus_id=None.

There already was a warning but this one is more explicit.

added P3 Quick Win labels

requested review from @babadie

assigned to @yschneider

added 3 commits

• 18a1b556 - 1 commit from branch master
• 0f0da119 - More explicit error message when corpus_id is not set in environment
• ceed94ca - Doc

Compare with previous version

I like the fact it becomes a property. So if someone tries to overwrite

merged