Currently, error messages for validation errors are produced in several
places:
* the high-level validator (which is built on the low-level one)
* scrub compaction and validation compaction (scrub in validate mode)
* scylla-sstable's validate operation
We plan to introduce yet another place which would use the low-level
validator and hence would have to produce its own error messages. To cut
down all this duplication, centralize the production of error messages
in the low-level validator, which now returns a `validation_result`
object instead of bool from its validate methods. This object can be
converted to bool (so its backwards compatible) and also contains an
error message if validation failed. In the next patches we will migrate
all users of the low level validator (be that direct or indirect) to use
the error messages provided in this result object instead of coming up
with one themselves.
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