Safely access optionals when calling DAO methods

[wslulciuc]

We currently use get() directly on optionals when accessing rows (though they technically should exist in most cases). For example:

final NamespaceRow namespaceRow = NamespaceDao.findNamespaceByName().get()

We should move towards safely accessing the row value returned and throw an exception when not present (where applicable):

import marquez.db.exceptions.RowNotFoundException;
.
.
final NamespaceRow namespaceRow =
  NamespaceDao.findNamespaceByName()
     .orElseThrow(() -> new RowNotFoundException())

The exception can be mapped to a 500.