[middleware] Don't try to access self.state.span in handle_error of Flask DB middleware if there is no current_app

[benhiller]

Both before_cursor_execute and after_cursor_execute exit early
if there is no current_app, but handle_error tries to finish
the span even if there is no current_app. This caused an
AttributeError when I was using beeline-python in the context of a
multi-threaded application, since when we used HoneyDBMiddleware off
of the thread it had been initialized on, self.state.span was never
set. We could probably also fix this by using getattr on self.state
before accessing span, but this seems more consistent with how the
other methods operate.

[tredman]

Thanks for the PR! 2.9.1 should be live within the next hour with your fix.

[benhiller]

Awesome, thanks for merging it!