feat: improve error message on unequal schemas during set ops

[NickCrews]

I got tired of ibis.union() failing but only saying "the schemas are different"

The relevant tests are in ibis/tests/expr/test_set_operations.py, looks like the bases are already covered pretty well, don't think I need to add any more

[cpcloud]

I'll rebase your PR to pick up the fix for the failing docs build.