refactor: Let CCoinsViewCache::BatchWrite return void

CCoinsViewCache::BatchWrite always returns true if called from a backed
cache, so just return void instead. Also return void from ::Sync and
::Flush.

This allows for dropping a FatalError condition and simplifying some
dead error handling code a bit.

Since we now no longer exercise the "error path" when returning from
`CCoinsView::BatchWrite`, make the method clear the cache instead. This
should only be exercised by tests and not change production behaviour.
This might slightly improve the coins_view fuzz test's ability to
generate better coverage.

Co-authored-by: l0rinc <pap.lorinc@gmail.com>

src/test/fuzz/coins_view.cpp

@@ -74,10 +74,10 @@ void TestCoinsView(FuzzedDataProvider& fuzzed_data_provider, CCoinsView& backend
                 }
             },
             [&] {
-                (void)coins_view_cache.Flush(/*will_reuse_cache=*/fuzzed_data_provider.ConsumeBool());
+                coins_view_cache.Flush(/*will_reuse_cache=*/fuzzed_data_provider.ConsumeBool());
             },
             [&] {
-                (void)coins_view_cache.Sync();
+                coins_view_cache.Sync();
             },
             [&] {
                 uint256 best_block{ConsumeUInt256(fuzzed_data_provider)};

src/test/fuzz/coinscache_sim.cpp

@@ -163,7 +163,7 @@ public:
     std::unique_ptr<CCoinsViewCursor> Cursor() const final { return {}; }
     size_t EstimateSize() const final { return m_data.size(); }
 
-    bool BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
+    void BatchWrite(CoinsViewCacheCursor& cursor, const uint256&) final
     {
         for (auto it{cursor.Begin()}; it != cursor.End(); it = cursor.NextAndMaybeErase(*it)) {
             if (it->second.IsDirty()) {
@@ -187,7 +187,6 @@ public:
                 }
             }
         }
-        return true;
     }
 };