Better error message for non-increasing index.

Attempting to write a non-increasing index version is most likely due to
parallel writes to the same symbol. We now indicate that in the error
message.

cpp/arcticdb/version/version_map.hpp

@@ -508,7 +508,7 @@ class VersionMapImpl {
             entry->validate();
 
         util::check(key.type() != KeyType::TABLE_INDEX || !entry->head_.has_value() || key.version_id() > entry->head_->version_id(),
-                    "Trying to write a non-increasing TABLE_INDEX key. New version: {}, Last version: {}",
+                    "Trying to write a non-increasing TABLE_INDEX key. New version: {}, Last version: {}. This is most likely due to parallel writes to the same symbol, which is not supported.",
                     key.version_id(), entry->head_ ? entry->head_->version_id() : VariantId{""});
         auto journal_key = to_atom(std::move(journal_single_key(store, key, entry->head_)));
         write_to_entry(entry, key, journal_key);