Failure to read default config file silently ignored

[arcresu]

In beets we noticed (see beetbox/beets#3264) that when confuse can't read the default config file it silently ignores the problem: https://github.com/beetbox/confuse/blob/04fd862963b0c1adb2cb81e4d97169869e88f2a9/confuse.py#L935-L939 Since beets was assuming that the file existed and was readable so that default config values were set, it led to some very obscure error messages about missing config fields.

This would have been a lot easier to debug if confuse threw an exception or something to indicate that the file was missing. Maybe some users of confuse don't need a default config file, but for those that do it seems that they would always expect it to be present.

[maddenfederico]

I'd like to bump this. Add on to that the fact that the optional parameter that sets this behavior isn't easily changeable by the user. There should be a distinction between default config files that don't exist (ignored) and those that fail to parse (should raise an exception).