Improve parser error message for SHOW VIEW statement

SHOW VIEW statement is actually not supported in Spark SQL. However, when a user quries such a statement, the error message is confusing. For example, the error message for

SHOW VIEWS IN my_database

is

missing 'FUNCTIONS' at 'IN'(line 1, pos 11)

which is misleading.
 

One possible way to fix is to explicitly capture these statements in a grammar rule and print user-friendly error message such as

SHOW VIEW statements are not supported.