google/externalaccount: return error messages from file cred opening

subjectToken function now return error messages when failing to open file credentials.

This will allow distinguishing between file not found and other errors.

google/externalaccount/filecredsource.go

@@ -26,7 +26,7 @@ func (cs fileCredentialSource) credentialSourceType() string {
 func (cs fileCredentialSource) subjectToken() (string, error) {
 	tokenFile, err := os.Open(cs.File)
 	if err != nil {
-		return "", fmt.Errorf("oauth2/google/externalaccount: failed to open credential file %q", cs.File)
+		return "", fmt.Errorf("oauth2/google/externalaccount: failed to open credential file %q due to %v", cs.File, err)
 	}
 	defer tokenFile.Close()
 	tokenBytes, err := ioutil.ReadAll(io.LimitReader(tokenFile, 1<<20))