Fix Msf::DBManager NameErrors

MSP-11152

Fix `NameError`s introduced by #4026 that were only revealed by integration test in Pro.

# Verification Steps

## metasploit-framework
### specs
- [ ] `rake spec`
- [ ] VERIFY no failures

## pro
- [ ] Ensure your msf3 symlink is pointing to this branch of metasploit-framework

### cucumber

#### DBImportError
- [ ] cucumber -p cinightly features/import.feature:12 features/import.feature:38 features/import.feature:54 features/import/import_3rd_party.feature:8
- [ ] VERIFY no error in task log.  Specifically, no `NameError` about `DBImportError`.

#### Zip::File
- [ ] cucumber -p cinightly features/discovery.feature:48
- [ ] VERIFY no error in task log.  Specifically, no `NameError` about `Zip::File`.