Fix #861 serde::Deserialize impl for Url prints confusing error message

[justy777]

Fixes #861

[denschub]

I think a better implementation would be

E::invalid_value(Unexpected::Other(&err_s), &self)

which then would keep the original "why it's broken" info intact:

    TOML parse error at line 2, column 12
      |
    2 | root_url = "http://[::1]:123456"
      |            ^^^^^^^^^^^^^^^^^^^^^
    invalid value: invalid port number, expected a string representing an URL

or even simpler:

Url::parse(s).map_err(E::custom)?;

which still would be fine:

    TOML parse error at line 2, column 12
      |
    2 | root_url = "http://localhost:8042111/blah"
      |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    invalid port number