Show intermediate errors in pinned tabs

[petemill]

Fix #8639
If loading a pinned tab errors, the error is not shown in the pinned tab, but is shown in a new tab which does not show the url which errors. The pinned tab then shows a non-branded error page. This can be a confusing UX as we've seen here, as it's not clear where the error tab has come from.

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed. (Ask a Brave employee to help if you cannot access this document.)

Test Plan:

On #8639 but also:

Steps to Reproduce

1. Use borked profile from bad session state can result in permanently borked profile #13261 in which www.ebay.com cannot be loaded due to infinite redirects due to bad cookies
2. Pin www.ebay.com
3. Restart browser

Actual result:
2 additional tabs are created with errors that do not specify which url or tab caused those errors.
Pinned ebay.com tab shows unbranded error.

Expected result:
Error is shown in pinned ebay tab, and upon resolving of the error (clearing cookies), and restarting, ebay continues to load in a pinned tab.
- verify that when the error is resolved, the pinned tab has remembered which url it is supposed to load

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #13610 into master will increase coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master   #13610      +/-   ##
==========================================
+ Coverage   56.74%   56.76%   +0.02%     
==========================================
  Files         285      285              
  Lines       28797    28782      -15     
  Branches     4755     4757       +2     
==========================================
- Hits        16341    16339       -2     
+ Misses      12456    12443      -13

Flag	Coverage Δ
#unittest	56.76% <0%> (+0.02%)	⬆️

Impacted Files	Coverage Δ
app/browser/tabs.js	23.93% <0%> (+0.14%)	⬆️