blockstorage: Return on fatal block file flush error

By returning an error code if `FlushBlockFile` fails, the caller now has
to explicitly handle block file flushing errors. Before this change
such errors were non-explicitly ignored without a clear rationale.

Prior to this patch `FlushBlockFile` may have failed silently in
`Chainstate::FlushStateToDisk`. Improve this with a log line. Also add a
TODO comment to flesh out whether returning early in the case of an
error is appropriate or not. Returning early might be appropriate to
prohibit `WriteBlockIndexDB` from writing a block index entry that does
not refer to a fully flushed block.

Besides `Chainstate::FlushStateToDisk`, `FlushBlockFile` is also called
by `FindBlockPos`. Don't change the abort behavior there, since we don't
want to fail the function if the flushing of already written blocks
fails. Instead, just document it.

src/validation.cpp

@@ -2511,7 +2511,11 @@ bool Chainstate::FlushStateToDisk(
                 LOG_TIME_MILLIS_WITH_CATEGORY("write block and undo data to disk", BCLog::BENCH);
 
                 // First make sure all block and undo data is flushed to disk.
-                m_blockman.FlushBlockFile();
+                // TODO: Handle return error, or add detailed comment why it is
+                // safe to not return an error upon failure.
+                if (!m_blockman.FlushBlockFile()) {
+                    LogPrintLevel(BCLog::VALIDATION, BCLog::Level::Warning, "%s: Failed to flush block file.\n", __func__);
+                }
             }
 
             // Then update all block file information (which may refer to block and undo files).