Be more specific about errors when adding pending contact
WIP because I still need to test these changes in reality, i.e. build a jar and play around with it. WIP because I might need to add more information for deleting a pending contact in the client.
I tested it while implementing error handling in Briar GTK (briar-gtk!97 (merged)).
Related to python-briar-wrapper!21 (merged).
Fixes #1825 (closed)