middleware/file: better notify error reporting
middleware/file: better notify error reporting

Report errors as originating from notifies in all cases.

Fixes #412

yongtang/coredns,chrisohaver/coredns,varyoo/coredns,coredns/coredns,miekg/coredns,johnbelamaric/coredns,miekg/coredns,jremond/coredns,jremond/coredns,ii/coredns,johnbelamaric/coredns,ii/coredns