Clarify error message when a .cert file is missing a corresponding key

The daemon uses two similar filename extensions to identify different
kinds of certificates. ".crt" files are interpreted as CA certificates,
and ".cert" files are interprted as client certificates. If a CA
certificate is accidentally given the extension ".cert", it will lead to
the following error message:

    Missing key ca.key for certificate ca.cert

To make this slightly less confusing, clarify the error message with a
note that CA certificates should use the extension ".crt".

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

registry/registry.go

@@ -109,7 +109,7 @@ func ReadCertsDirectory(tlsConfig *tls.Config, directory string) error {
 			keyName := certName[:len(certName)-5] + ".key"
 			logrus.Debugf("cert: %s", filepath.Join(directory, f.Name()))
 			if !hasFile(fs, keyName) {
-				return fmt.Errorf("Missing key %s for certificate %s", keyName, certName)
+				return fmt.Errorf("Missing key %s for client certificate %s. Note that CA certificates should use the extension .crt.", keyName, certName)
 			}
 			cert, err := tls.LoadX509KeyPair(filepath.Join(directory, certName), filepath.Join(directory, keyName))
 			if err != nil {
@@ -122,7 +122,7 @@ func ReadCertsDirectory(tlsConfig *tls.Config, directory string) error {
 			certName := keyName[:len(keyName)-4] + ".cert"
 			logrus.Debugf("key: %s", filepath.Join(directory, f.Name()))
 			if !hasFile(fs, certName) {
-				return fmt.Errorf("Missing certificate %s for key %s", certName, keyName)
+				return fmt.Errorf("Missing client certificate %s for key %s", certName, keyName)
 			}
 		}
 	}