better handling of file locking

[psifertex]

Right now due to the sqlite library doing the initial BNDB load only one copy of BN at a time can make writes at a time. However, opening a new process for the same database doesn't tell the user this will be a problem until they try to save and then things end badly.

Instead, BN should either:

1. Warn the user that the file is unavailable for opening, explaining how to reset the lock in extreme instances where another process has crashed, or explain that the other copy must be terminated before the database can be written to again
2. Have a "read-only" view of a BNDB that allows the file to be opened but no changes made to any analysis (this seems harder, but potentially more valuable

This is partly an enhancement but partially a bug because right now this can result in lost data in some situations.