Linkcheck: Fix broken url not reporting error

[TheTripleV]

Subject: Linkcheck: Fix broken url not reporting error

Feature or Bugfix

• Bugfix

Purpose

Some links are printed as broken but do not error out the build.
This issue appeared when we include tel: links in our build.

Relates

• Linkcheck is failing on phone numbers #8321

[tk0miya]

I think tel: hyperlink should not be treated as an error in linkcheck. But it seems your PR treated them as errors. Is this expected?

[tk0miya]

Note: I just made a PR #8332 to make tel: links ignored.

[TheTripleV]

This PR only addresses the issue where unknown schemas are reported as broken by linkcheck but do not fail the sphinx build.

Here is a CI run showing the issue:

1. linkcheck does not recognize the tel: schema and prints out (broken)
   https://github.com/wpilibsuite/frc-docs/runs/1276228243#step:6:601

2. The build still succeeds (even though it shouldn't)
   https://github.com/wpilibsuite/frc-docs/runs/1276228243#step:6:752

[tk0miya]

Okay, I understand. It would be better to merge this to handle an error by unknown schema. Additionally, it would be better to suppress the error of tel schema.

[tk0miya]

Merged. Thank you for your contribution!