Twisted web Resources should "return ErrorPage" instead of "raise WebError"

[sajith]

In case of errors, twisted.web Resource instances should be responding with an error page, instead of throwing an exception. Any raise WebError instances we have in Tahoe WUI should be replaced with return ErrorPage.

More background is in ​this discussion at GitHub about a PR that solves #3382.

Sub-tickets:

• #3422 - allmydata/web/directory.py should return error pages instead of raising WebError
• #3423 - allmydata/web/filenode.py should return error pages instead of raising WebError
• #3424 - allmydata/web/operations.py should return error pages instead of raising WebError
• #3425 - allmydata/web/root.py should return error pages instead of raising WebError
• #3426 - allmydata/web/status.py should return error pages instead of raising WebError
• #3427 - allmydata/web/unlinked.py should return error pages instead of raising WebError
• #3429 - allmydata/web/check_result.py should return error pages instead of raising WebError

[exarkun]

This is probably worth splitting up. The changes will be spread out across a lot of the web code and have little overlap (apart from a few shared error page helpers that will come pretty early, I suspect).

[exarkun]

This is somewhat mitigated by the work for ticket:3422 which introduced the @render_exception decorator which will transform an exception into an ErrorPage?.

This may still be a sensible change to make but we don't need it to fully escape Nevow at this point.

Though all the currently linked sub-tickets are resolved there are still some parts of the web code that do raise WebError. I think we just didn't get around to filing tickets for them.

[itamarst]

Ticket retargeted after milestone closed