[SPARK-15029] improve error message for Generate

## What changes were proposed in this pull request?

This PR improve the error message for `Generate` in 3 cases:

1. generator is nested in expressions, e.g. `SELECT explode(list) + 1 FROM tbl`
2. generator appears more than one time in SELECT, e.g. `SELECT explode(list), explode(list) FROM tbl`
3. generator appears in other operator which is not project, e.g. `SELECT * FROM tbl SORT BY explode(list)`

## How was this patch tested?

new tests in `AnalysisErrorSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #12810 from cloud-fan/bug.