Sets "Pywb Error" string as translatable in error template

[Quirinus]

Description

Sets the "Pywb Error" string in the <h2> heading as translatable in the error.html template.

Motivation and Context

• The string "Pywb Error" isn't translatable in this line.
• Fixes String not set as translatable in template #867

Screenshots (if appropriate):

The "Pywb Error" isn't translated, but this pull request fixes that.

Types of changes

• Replay fix (fixes a replay specific issue)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added or updated tests to cover my changes.
• All new and existing tests passed.

I have no idea how to run the tests, but hopefully just making a string translatable shouldn't break anything.

[edsu]

Thanks for noticing and fixing this!

[tw4l]

Thank you for the fix!

[tw4l]

Confirmed that the test failure seen in CI is happening on main and not related to this change (thought so but wanted to make sure!), so I'm going to go ahead and merge this in and resolve that separately.