If a docstring fails assertion checks, output the offending docstring… (#458)

* If a docstring fails assertion checks, Output the offending docstring to help the user pinpoint the problem.

* Update docstrings.py

Co-authored-by: Maximilian Hils <git@maximilianhils.com>

Author: vsajip

pdoc/docstrings.py

@@ -104,9 +104,8 @@ def _indented_list(contents: str) -> list[str]:
     ]
     """
     # we expect this to be through cleandoc() already.
-    assert not contents.startswith(" ")
-    assert not contents.startswith("\n")
-    assert "\t" not in contents
+    assert not contents.startswith(" "), contents
+    assert not contents.startswith("\n"), contents
 
     ret: list[str] = []
     for line in contents.splitlines(keepends=True):