TurboGears2 migration: update ini files for error email settings

Error emails are now handled by backlash, which is configured through
TurboGears2's ErrorReporter. ErrorReporter expects different configuration
key names than Pylons did, moreover under a new 'trace_errors' namespace.

Since some of the email-related settings are shared between application and
error emails, we cannot just rename the existing settings (it would be very
odd to have application settings under a 'trace_errors' namespace).
Requiring the user to duplicate its settings is also undesirable.

Instead, use 'get' to populate the trace_errors namespace based on the
existing settings we already had. Unfortunately, 'get' expects the setting
to be actually present, so we need to provide an out-of-the-box value for
the error-related email settings or there will be an error at startup. We
use empty values because there is no realistic default value we can provide.