Improve serde error output

[konstin]

In our tests using the serde impl on Url, we found that failing URL serialization says the inverse of what it should say.

[project]
name = "foo"
version = "0.0.0"
dependencies = [
  "tqdm ==4.66.0",
]
[tool.uv.sources]
tqdm = { url = "§invalid#+#*Ä" }

error: Failed to parse: `pyproject.toml`
  Caused by: TOML parse error at line 10, column 16
   |
10 | tqdm = { url = "§invalid#+#*Ä" }
   |                ^^^^^^^^^^^^^^^^^
invalid value: string "§invalid#+#*Ä", expected relative URL without a base

It says that expected a relative URL without a base, when this was the unexpected input that caused the error. Using serde::de::Error::custom oder serde::de::Error::invalid_value improves the error message:

error: TOML parse error at line 8, column 16
  |
8 | tqdm = { url = "§invalid#+#*Ä" }
  |                ^^^^^^^^^^^^^^^^^
relative URL without a base: "§invalid#+#*Ä"

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@ebd5cfb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
url/src/lib.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #982   +/-   ##
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  Coverage        ?   82.35%           
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  Files           ?       22           
  Lines           ?     3559           
  Branches        ?        0           
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  Hits            ?     2931           
  Misses          ?      628           
  Partials        ?        0           

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