Fix getLayoutName to return "n/a" in case we lack information

In case if xkbd layout information is not avaliable kbdd returns
error without setting error message. This commit fix kbdd to
return "n/a" instead of silent error. Another approach is no set
error.

X-GITHUB-BUG: #27
X-GITHUB-BUG-URL: https://github.com/qnikst/kbdd/issues/27