plugin_cache: restore proper error handling in cache initialization

The race condition fix in d943aba inadvertently changed error handling
behavior by silently ignoring cache initialization failures. This could
lead to degraded performance when the cache fails to initialize.

Restore proper error propagation while maintaining thread safety using
sync.Once and a captured error variable.

Author: jedisct1

dnscrypt-proxy/plugin_cache.go

@@ -3,6 +3,7 @@ package main
 import (
 	"crypto/sha512"
 	"encoding/binary"
+	"fmt"
 	"sync"
 	"time"
 
@@ -144,12 +145,18 @@ func (plugin *PluginCacheResponse) Eval(pluginsState *PluginsState, msg *dns.Msg
 		expiration: time.Now().Add(ttl),
 		msg:        *msg,
 	}
+	var cacheInitError error
 	cachedResponses.cacheOnce.Do(func() {
 		cache, err := sievecache.NewSharded[[32]byte, CachedResponse](pluginsState.cacheSize)
-		if err == nil {
+		if err != nil {
+			cacheInitError = err
+		} else {
 			cachedResponses.cache = cache
 		}
 	})
+	if cacheInitError != nil {
+		return fmt.Errorf("failed to initialize the cache: %w", cacheInitError)
+	}
 	if cachedResponses.cache != nil {
 		cachedResponses.cache.Insert(cacheKey, cachedResponse)
 	}