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/lib/util.py

@@ -70,7 +70,8 @@ async def run_curl(args: list[str], parse_json: bool=False, wd=None, throw=None,
     return ret, stderr, status
 
 async def run_askpass(askpass_env: list[str], key: AskpassKey, host: str|None=None, ec: ExecContext|None=None):
-    assert host is None # Currently unsupported
+    if host is not None: # Currently unsupported
+        raise NotImplementedError(f'Tried to run askpass with host "{host}"')
     for var in askpass_env:
         exe = os.getenv(var)
         if exe is None: