App.__format_topdir(): Mention relative in error

__format_topdir() accepts "absolute", "relative", "unaltered", and
"make:<variable-name>", but the error message it raises for anything
else only lists "unaltered", "absolute", and
"make:<variable-name>", leaving out "relative".

Add "relative" to the list of valid formats in the error message.

Assisted-by: unsloth/Qwen3.8-27B-GGUF:Q4_K_M with pi.dev v0.84.2
Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/App.py

@@ -61,7 +61,8 @@ class App(Base):
                 if m is None:
                     raise Exception(
                         f'Can\'t interpret "{fmt}" as valid topdir reference, '
-                        'expecting "unaltered", "absolute", or "make:<variable-name>"'
+                        'expecting "absolute", "relative", "unaltered", '
+                        'or "make:<variable-name>"'
                     )
                 return '$(' + m.group(1) + ')'