Fix errors when closing

[rkistner]

Fixes #36.

Also improves handling of database initialization errors and other uncaught errors in connection isolates.

The new test currently often fails with this error:

SqliteException(261): while executing, database is locked, database is locked (code 261)
    Causing statement: PRAGMA synchronous = NORMAL, parameters: 

It is likely caused by opening multiple connections at the same time. #34 will fix the issue.

[rkistner]

Closing in favor of #38, which builds on this PR. The "closing database" tests are still unstable on this version (even though it's way better than before), while it appears to be completely fixed in #38.