lib: Fix silent assertitons

There are a couple of assert statements in the codebase which can make jw-pkg fail without any detail whatsoever if --backtrace is not specified, fix that.
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/cmds/projects/CmdListRepos.py

@@ -59,7 +59,7 @@ class CmdListRepos(Cmd): # export
                         '-H', 'X-GitHub-Api-Version: 2022-11-28',
                     ]
                     if password is not None:
-                        assert username is not None
+                        assert username is not None, f'Assertion failed: username is empty but password isn\'t for "{args.base_url}"'
                         cmd_input = (f'-u {username}:{password}').encode('utf-8')
                         curl_args.extend(['-K-'])
                     curl_args.append(f'https://api.github.com/users/{args.from_owner}/repos')