Exceptions when loading corrupted data

[JelleAalbers]

This improves strax' behaviour under exceptional conditions:

• Raise DataCorrupted when attempting to load data which has not been completely written, or whose writing terminated with an exception. This check is skipped only for metadata loading, so we can still inspect the exception that corrupted the data. (thanks @feigaodm for demonstrating how necessary this exception is)
• Raise a more informative exception when a user attempt to load data with array_fields through get_df (thanks @coderdj)
• Remove unused code for handling exceptions on ambiguous data requests (which can occur with fuzzy data matching). None of the backends actually supported this. Ambiguous requests will continue to simply return the first match. Note such requests are only possible if the user sets fuzzy matching options.
• Bugfix for fuzzy matching in the files backend (wrong directory name returned). I added a test to catch this in the future.