Clearer error message for URL without a hostname.

[bitdancer]

When I got the existing error message I had no idea what it meant. A little digging in the code reveals that the problem was I provided a URL that did not contain a hostname, so aiohttp was unable to parse the hostname out of that URL. Here is an alternate wording for the error message that I hope will lead the library user to understand what went wrong more quickly.

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep alphabetical order, the file is sorted by names.
• Add a new entry to CHANGES.rst
  • Choose any open position to avoid merge conflicts with other PRs.
  • Add a link to the issue you are fixing (if any) using #issue_number format at the end of changelog message. Use Pull Request number if there are no issues for PR or PR covers the issue only partially.

[fafhrd91]

could you create PR against https://github.com/aio-libs/aiohttp

[lock]

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