Print meaningful config error message

[vkrizan]

Not just plain: "ERROR: Could not read configuration file, using defaults"

The message format is the same as for ValueError used down below (for key validation).

[vkrizan]

@gravitypriest Would you mind having a look. Thank you.

[vkrizan]

@psachin can you please have a look? Thanks!

[candlepin-bot]

Can one of the admins verify this patch?

[vkrizan]

#stale