Log invalid block hash to make debugging easier.

src/main.cpp

@@ -3371,7 +3371,7 @@ static bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state
             if (ppindex)
                 *ppindex = pindex;
             if (pindex->nStatus & BLOCK_FAILED_MASK)
-                return state.Invalid(error("%s: block is marked invalid", __func__), 0, "duplicate");
+                return state.Invalid(error("%s: block %s is marked invalid", __func__, hash.ToString()), 0, "duplicate");
             return true;
         }