Emit an error instead of doing ASSERT


mysql-test/r/create_not_windows.result:
  update result
mysql-test/t/create_not_windows.test:
  Add test case for bug #16532: mysql server assert in debug if table definition is removed
sql/sql_table.cc:
  it could happen that an user deletes the frm file on OS level.
  In addition if the table was already opened, the file system level
  check for existance of table.frm will pass but get_cached_table_share()
  will return non-NULL value. In this case we will emit the standard
  error that the table exist. The error case is quite obscure but error
  message is better than a sigabort.