url: remove traceback from debug log when fetching URL fails

[half-duplex]

Description

If url.py can't connect to a server because of things like certificate issues (https://expired.badssl.com/), DNS (http://fake.host.local/), etc, it currently dumps most of a page of exception tracebacks into logs. This is useful to nobody, and it shouldn't do that.

This patch replaces the page of tracebacks with the following:
sopel.modules.url DEBUG - Unable to reach URL: 'http://fake.host/': HTTPConnectionPool(host='fake.host', port=80): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f95ef238340>: Failed to establish a new connection: [Errno -2] Name or service not known'))
I think that's quite sufficient for troubleshooting without being excessively spammy.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches

[Exirel]

1. language
2. a title that makes sense

Thank you.

[dgw]

@Exirel Changing the title doesn't modify the message of the commit that gets merged, though. (It makes sense to me, though, so I don't mind keeping it.)

[Exirel]

I'm still annoyed by "vomit unsurprising exception", because 1. it's not "vomiting" anything, it outputs a debug traceback, and 2. it's not unsurprising at all.

[dgw]

Then next time you can request the commit message be modified to match your new PR title. 😃