Add a protected file to the authplain config cascade

This adds the possibility to define file with protected users in the
config cascade to be used with the plain auth mechanism.

This makes it poss

Add a protected file to the authplain config cascade

This adds the possibility to define file with protected users in the
config cascade to be used with the plain auth mechanism.

This makes it possible for farms to have way to inherit Farmer users in
animals.

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Report more meaningful errors when an auth backend fails. closes #1093