Add details to the EOF exceptions

[morozov]

The additional details should simplify troubleshooting EOF exceptions in the binlog connector (#15, #50). By knowing the details, one can replicate the binlog to the local development environment and examine it at the position of failure. For instance, one can confirm that the binlog is intact, and the root cause of the issue is somewhere else.

Example error messages:

Failed to read remaining 28 of 36 bytes from position 1596693181. Block length: 7347. Initial block length: 8133.

or

Failed to read next byte from position 20131056

[morozov]

@osheroff any chance to get this reviewed?