Take an improvement pass over validation errors

[TedTed]

There's room for improvement in many of our validation errors. @Maegereg points out two common patterns:

• Describing the error in ways that might be hard for a user to understand (e.g. f"Nonexistent columns in get_groups query: {nonexistent_columns}" - "nonexistent column" could mean a few different things).
• Not identifying the part of the query causing the issue (e.g. f"Map must set augment=True to ensure that ID column '{input_schema.id_column}' is not lost." could be hard to diagnose if you have multiple maps).

This issue is to go through all such errors and make sure that each one leads to an error message that can be understood by a user who doesn't know the details of our code (in particular, they're not ever interacting with QueryExpr directly, only with QueryBuilder methods), and — whenever possible — gives clear instructions on how to fix the error.