Improve errors when loading configuration

Context

Currently, any error which occurs when loading configuration leads to a log message of Permission Denied when reading config file. Aborting. (https://gitlab.com/BuildGrid/buildbox/userchroot/-/blob/master/userchroot.c#L649). This is confusing in cases where the config file permissions are fine but there is still a config-related issue, such as no entry for the expected user/path.

There should be some more specific error messages when parsing the config. In the case of a missing entry, the error should ideally contain the user/path combination that was expected.

Acceptance Criteria

• Unit tests