lib: handle DatabaseModifiedError in _n_message_ensure_metadata

The retries are hardcoded to a small number, and error handling aborts
than propagating errors from notmuch_database_reopen. These are both
somewhat justified by the assumption that most things that can go
wrong in Xapian::Database::reopen are rare and fatal. Here's the brief
discussion with Xapian upstream:

   24-02-2017 08:12:57 < bremner> any intuition about how likely
      Xapian::Database::reopen is to fail? I'm catching a
      DatabaseModifiedError somewhere where handling any further errors is
      tricky, and wondering about treating a failed reopen as as "the
      impossible happened, stopping"

   24-02-2017 16:22:34 < olly> bremner: there should not be much scope for
    failure - stuff like out of memory or disk errors, which are probably a
    good enough excuse to stop

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