ParseDb-select gives obtuse error for missing fields

[Jason Vander Heiden]

If you pass a missing field to ParseDb-select it still tries to check the field, resulting in the confusing error:

    return _compile(pattern, flags).search(string)
TypeError: expected string or bytes-like object

We should add a check for missing fields and output an informative message.

[Jason Vander Heiden]

Fixed in 5b0de02