Use error interfaces for content/metadata

These interfaces allow us to preserve both the checking of error "cause"
as well as messages returned from the gRPC API so that the client gets
full error reason instead of a default "metadata: not found" in the case
of a missing image.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

content/locks.go

@@ -1,9 +1,8 @@
 package content
 
 import (
+	"fmt"
 	"sync"
-
-	"github.com/pkg/errors"
 )
 
 // Handles locking references
@@ -20,7 +19,7 @@ func tryLock(ref string) error {
 	defer locksMu.Unlock()
 
 	if _, ok := locks[ref]; ok {
-		return errors.Wrapf(ErrLocked, "key %s is locked", ref)
+		return ErrLocked(fmt.Sprintf("key %s is locked", ref))
 	}
 
 	locks[ref] = struct{}{}