R1:0ed496de2282

Throw an exception if `local.json` can't be read

Summary: Our `local.json` configuration file contains various secrets, including database usernames and passwords. As such, we recently changed the permissions on this file from `0644` to `0640`. After doing so, however, I constantly forget to run commands with `sudo`. This is made worse by the fact that `PhabricatorConfigLocalSource` seems to simply ignore `local.json` is it isn't readable, whereas throwing an `Exception` would have saved me a lot of…