Update HSD error block schema to conform to >=v1.1 HSD spec

HSD error block has longer 4-byte block_length as of HSD format v1.1, which could result in corrupted imagery in extreme error cases. Other blocks have lengths that are 2 bytes.

• libHimawari needs a non-urgent fix to error block_length field size, it's 4-bytes as far back as version 1.1 of the HSD spec.
• this only impacts libHimawari once that error block block_length gets beyond 16 bits, beyond which point imagery will be significantly corrupted
• the little-endianness of the file and infrequency of a large error block are likely masking the problem, given libhimawari's block_length obedience in traversing my memory map, along with block_number confirmation

• update ahi_hsf.py
• regenerate himawari_file_struct.gxx using updated python code
• regression test