build: remove error return from shardName method

Change-Id: I42b282c9898d1c204fbdcfabf20bb4dc2a650228
diff --git a/build/builder.go b/build/builder.go
index 048b3b3..08db631 100644
--- a/build/builder.go
+++ b/build/builder.go
@@ -142,22 +142,20 @@
 }
 
 // ShardName returns the name the given index shard.
-func (o *Options) shardName(n int) (string, error) {
+func (o *Options) shardName(n int) string {
 	abs := url.QueryEscape(o.RepositoryDescription.Name)
 	if len(abs) > 200 {
 		abs = abs[:200] + hashString(abs)[:8]
 	}
 	return filepath.Join(o.IndexDir,
-		fmt.Sprintf("%s_v%d.%05d.zoekt", abs, zoekt.IndexFormatVersion, n)), nil
+		fmt.Sprintf("%s_v%d.%05d.zoekt", abs, zoekt.IndexFormatVersion, n))
 }
 
 // IndexVersions returns the versions as present in the index, for
 // implementing incremental indexing.
 func (o *Options) IndexVersions() []zoekt.RepositoryBranch {
-	fn, err := o.shardName(0)
-	if err != nil {
-		return nil
-	}
+	fn := o.shardName(0)
+
 	f, err := os.Open(fn)
 	if err != nil {
 		return nil
@@ -268,11 +266,7 @@
 	for {
 		shard := b.nextShardNum
 		b.nextShardNum++
-		name, err := b.opts.shardName(shard)
-		if err != nil {
-			break
-		}
-
+		name := b.opts.shardName(shard)
 		if err := os.Remove(name); os.IsNotExist(err) {
 			break
 		}
@@ -428,10 +422,7 @@
 		}
 	}
 
-	name, err := b.opts.shardName(nextShardNum)
-	if err != nil {
-		return nil, err
-	}
+	name := b.opts.shardName(nextShardNum)
 
 	shardBuilder, err := b.newShardBuilder()
 	if err != nil {