Add a more explicit error message when paper is already in HAL
Add a more explicit error message when paper is already in HAL

And more generally, always try to return a verbose error message when HAL returns a 400 error code.

Closes #356.

dissemin/dissemin,wetneb/dissemin,dissemin/dissemin,dissemin/dissemin,wetneb/dissemin,dissemin/dissemin,dissemin/dissemin,wetneb/dissemin,wetneb/dissemin