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).