[funfuzz] Don't fail with an exception when hitting a rev that failed compilation in the cache

[nth10sd]

Traceback (most recent call last):
  File "/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/local/Cellar/python/2.7.14/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/Users/fuzz7/trees/venv-fm/lib/python2.7/site-packages/funfuzz/bot.py", line 283, in <module>
    main()
  File "/Users/fuzz7/trees/venv-fm/lib/python2.7/site-packages/funfuzz/bot.py", line 125, in main
    build_info = ensureBuild(options)
  File "/Users/fuzz7/trees/venv-fm/lib/python2.7/site-packages/funfuzz/bot.py", line 194, in ensureBuild
    compile_shell.obtainShell(cshell, updateLatestTxt=updateLatestTxt)
  File "/Users/fuzz7/trees/venv-fm/lib/python2.7/site-packages/funfuzz/js/compile_shell.py", line 652, in obtainShell
    raise Exception("Found a cached shell that failed compilation...")
Exception: Found a cached shell that failed compilation...
Something went wrong when calling: ['/Users/fuzz7/trees/venv-fm/bin/python', '-u', '-m', 'funfuzz.bot', '-b', '--random', '--target-time', '28800']

We should probably handle the exception a little better.