fix: add explicit error for invalid versions encountered during colle…

…ct (#101)

* Add explicit error for invalid versions encountered during collect

* Adjust line length to appease the linter

src/scriv/collect.py

@@ -71,6 +71,12 @@ def collect(
                 if etitle is None:
                     continue
                 eversion = Version.from_text(etitle)
+                if eversion is None:
+                    sys.exit(
+                        f"Entry {etitle!r} is not a valid version! "
+                        + "If scriv should ignore this heading, add "
+                        + "'scriv-end-here' somewhere before it."
+                    )
                 if eversion == version:
                     sys.exit(
                         f"Entry {etitle!r} already uses "