The `fsbridge::get_filesystem_error_message()` function exhibits several
drawbacks:
1. It was introduced in https://github.com/bitcoin/bitcoin/pull/14192 to
account for platform-specific variations in
`boost::filesystem::filesystem_error::what()`. Since migrating to
`std::filesystem`, those discrepancies no longer exist.
2. It fails to display UTF-8 paths correctly on Windows.
3. It relies on `std::wstring_convert`, which was deprecated in C++17
and removed in C++26.
This change removes the `fsbridge::get_filesystem_error_message()`
function, thereby resolving all of the above issues.
Additionally, filesystem error messages now use the "Warning" log level.
BerkeleyRODatabase is intended for use after BDB is removed, so it needs
to be able to read all of the records from a BDB file. Thus an
independent deserializer for BDB data files is implemented in it. This
deserializer is targeted towards the data files that Bitcoin Core
creates so it does not fully support all of BDB's features (e.g. other
database types, encryption, etc.).
Implement ReadKey and HasKey of BerkeleyROBatch, and Next of BerkeleyROCursor.
Also adds the containers for records to BerkeleyRODatabase so that
BerkeleyROBatch will be able to access the records.
