Summary:
Fix for issue https://github.com/facebook/rocksdb/issues/6316

When an append/sync of the manifest file fails due to an IO error such
as NoSpace, we don't always put the DB in read-only mode. This is true
for flush and compactions, as well as foreground operatons such as column family
add/drop, CompactFiles etc. Subsequent changes to the DB will be
recorded in the same manifest file, which would have a corrupted record
in the middle due to the previous failure. On next DB::Open(), it will
fail to process the full manifest and data will be lost.

To fix this, we reset VersionSet::descriptor_log_ on append/sync
failure, which will force a new manifest file to be written on the next
append.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6331

Test Plan: Add new unit tests in error_handler_test.cc

Differential Revision: D19632951

Pulled By: anand1976

fbshipit-source-id: 68d527cb6e59a94cbbbf9f5a17a7f464381d51e3