Add a "Proceed to destination" button when pulling fails

[yuvipanda]

Proposed change

Sometimes, pulling fails (like in #277) but we still want the user to proceed to the target URL. We should add a button that shows up and allows you to go past nbgitpuller and to the target URL, and have this button only show up when pulling fails.

Alternative options

Currently users have to manually edit the URL to go to their target, which is very unideal.

[ericvd-ucb]

At least there could be a general button that goes to https://datahub.berkeley.edu/hub/user-redirect/tree