Don't throw exception when ly files not found

[SilverRainZ]

$ make html
Running Sphinx v3.5.4
loading translations [zh_CN]... done
loading pickled environment... done
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 3 source files that are out of date
updating environment: 3 added, 15 changed, 0 removed
reading sources... [ 72%] notes/music-theory/solfeggio-lesson-of-daluobei/index                                                                                                  
Exception occurred:
  File "/home/la/.local/lib/python3.9/site-packages/sphinxnotes/lilypond/__init__.py", line 115, in read_lily_source
    with open(lilyfn, 'r') as f:
FileNotFoundError: [Errno 2] 没有那个文件或目录: '/home/la/documents/bullet/notes/music-theory/solfeggio-lesson-of-daluobei/./major2-minor2.ly'
The full traceback has been saved in /tmp/sphinx-err-obuvrfi8.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
make: *** [Makefile:38：html] 错误 2