Database error checking

[DL6ER]

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Improve error checking in the database routines to cover all actions that can fail, e.g., due to readonly database errors.

• Check if the pihole-FTL.db database is still available at the beginning of all database routines in database/common.c
• Log if we failed to update the database from version 3 -> 4 and return early if this is the case.
• Simplify logging in case of an error.
  Previously:

  [2019-07-06 17:00:55.526 32339] SQLite3 message: statement aborts at 36: [CREATE UNIQUE INDEX network_hwaddr_idx ON network(hwaddr)] attempt to write a readonly database (8)
  [2019-07-06 17:00:55.526 32339] dbquery(CREATE UNIQUE INDEX network_hwaddr_idx ON network(hwaddr)) - SQL error (8): attempt to write a readonly database
  [2019-07-06 17:00:55.526 32339] check_database(8): Disabling database connection due to error
  [2019-07-06 17:00:55.526 32339] unify_hwaddr(): "CREATE UNIQUE INDEX network_hwaddr_idx ON network(hwaddr)" failed!
  

  Now:

  [2019-07-06 17:00:55.526 32339] SQLite3 message: statement aborts at 36: [CREATE UNIQUE INDEX network_hwaddr_idx ON network(hwaddr)] attempt to write a readonly database (8)
  [2019-07-06 17:00:55.526 32339] check_database(8): Disabling database connection due to error
  [2019-07-06 17:00:55.526 32339] ERROR: unify_hwaddr() failed!