[Buildbot] #2991: Confusing error message when the database is missing or empty

[Buildbot trac]

#2991: Confusing error message when the database is missing or empty
----------------------+-----------------------
Reporter:  sa2ajj     |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.8.9      |   Keywords:
----------------------+-----------------------
 When the database specified in `c['db']` does not exist or empty, the user
 gets a confusing error message:

 {{{
 The Buildmaster database needs to be upgraded before this version of
 buildbot can run.  Use the following command-line

     buildbot upgrade-master path/to/master

 to upgrade the database, and try starting the buildmaster again. You may
 want to make a backup of your buildmaster before doing so.
 }}}

 One of the users suggested on
 [http://irclogs.jackgrigg.com/irc.freenode.net/buildbot/2014-10-31#i_3484303
 IRC] that `buildbot start` should stop and complain about the missing
 database as it does about missing `master.cfg`

 On a related note (maybe it's worth a separate ticket), after Buildbot
 complains about it, there's a SQLite database created in the master
 directory, which added to the confusion (and the file mode is different
 from what `create-master` produces: 0600 (former) vs 0644 (latter)).

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Ticket URL: <http://trac.buildbot.net/ticket/2991>
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