Fix unnamed append points

In `ConfigFile.compile()`, the variable `content` is used but not
initialized, which results in unnamed append points being deleted and
`insert_lines` being appended at the end instead. Due to the
undifferentiated `try/except` block around this, the error is silently
ignored.

This fix makes the `try/except` block only handle `ValueError`, which
would occur when there's no explicit append point.