Skip importing a PAGE XML transcript if there are no valid regions to create

While the root cause of #175 (closed) is skipping every polygon, there could be edge cases where every polygon should be skipped because the XML is completely wonky. In these cases, we should not even try to call CreateElements and get this error:

[ERROR] Failed importing transcript 1328264/Copy_of_HTR_Validation_Set_'NorHand_pylaia'/page/no-nb_digimanus_147901_0001.xml: 400 - {'elements': {'non_field_errors': ['This list may not be empty.']}}

We can raise a more explicit error to make it clear that no regions could be imported. When no regions are imported, we cannot import any text lines or transcriptions, so we can just crash here.