To improve debuggability, we need to propagate original error messages
from Paxos verbs to the user. This change adds constructors that take
an error message directly, enabling better error reporting.
Additionally, functions such as write_timeout_to_read,
write_failure_to_read etc are updated to use these message-based
constructors. These functions are used in storage_proxy::cas to
convert between different error types, and without this change,
they could lose the original error message during conversion.
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