tsdb should return more details when it fails to open the db

[simonpasquier]

Bug Report

What did you do?

Start Prometheus with a data dir containing an invalid meta.json file.

What did you expect to see?

A meaningful log message.

What did you see instead? Under which circumstances?

No indication of where to look at. See logs below.

Environment

• Prometheus version:

prometheus, version 2.2.1 (branch: master, revision: 416db814e85f6547ad8c7b0cc067022a1a804479)
  build user:       simon@simon-laptop
  build date:       20180425-08:56:22
  go version:       go1.10.1

• Logs:

level=error ts=2018-04-25T08:57:09.900515419Z caller=main.go:593 err="Opening storage failed unexpected end of JSON input"

[lock]

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