[SPARK-20916][SQL] Improve error message for unaliased subqueries in FROM clause

## What changes were proposed in this pull request?

We changed the parser to reject unaliased subqueries in the FROM clause in SPARK-20690. However, the error message that we now give isn't very helpful:

    scala> sql("""SELECT x FROM (SELECT 1 AS x)""")
    org.apache.spark.sql.catalyst.parser.ParseException:
    mismatched input 'FROM' expecting {<EOF>, 'WHERE', 'GROUP', 'ORDER', 'HAVING', 'LIMIT', 'LATERAL', 'WINDOW', 'UNION', 'EXCEPT', 'MINUS', 'INTERSECT', 'SORT', 'CLUSTER', 'DISTRIBUTE'}(line 1, pos 9)

We should modify the parser to throw a more clear error for such queries:

    scala> sql("""SELECT x FROM (SELECT 1 AS x)""")
    org.apache.spark.sql.catalyst.parser.ParseException:
    The unaliased subqueries in the FROM clause are not supported.(line 1, pos 14)

## How was this patch tested?

Modified existing tests to reflect this change.

Author: Liang-Chi Hsieh <viirya@gmail.com>

Closes #18141 from viirya/SPARK-20916.