Add variable rocksdb_ignore_error to skip error at startup

In some cases error avoids to let RocksDB start.
In this cases the only options is to restore a full backup , even if
the error is not critical like

[ERROR] RocksDB: Could not get index information for Index Number, (x,x) , table tablename
[ERROR] RocksDB: Failed to initialize DDL mananger
[ERROR] Plugin 'ROCKSDBÄ init function returned error.

So in some cases there should be a possibility to skip the error.

Suggestion:
Add variable

rocksdb_ignore_error

Simple Version wood be a boolean, maybe it makes also sense to create a ENUM for different error types.
Certainly default should be 0 or 'NONE'