Show better error message for 413 from nginx

[matthew-white]

Before sending a file, Frontend checks that the file doesn't exceed the nginx limit of 100 MB. However, Frontend only checks that for files, not other request bodies, so it's possible for other requests to result in a 413 error from nginx. (Backend itself never seems to return a 413, though I think it should when the bodyParser limit of 250 kB is exceeded: #788.) I don't think I've previously seen a 413 before, but with getodk/central-backend#589 on the way, it now seems very possible.

Currently, if Frontend receives a 413 from nginx, it shows an error message of "Something went wrong: error code 413." It'd be better if it showed a more informative message. In other words, unlike files, we'll allow other large request bodies to be sent to nginx, but then we'll have Frontend show an informative error message for the resulting 413. We thought about having Frontend just not send the request in the first place, but we discussed the possibility that a user might want to increase the limit. Due to getodk/central-backend#609, if we had Frontend enforce the limit, then the user would have to modify Frontend code, which we don't want. We can have Frontend enforce the limit in the future, once getodk/central-backend#609 is resolved: see #781.

[matthew-white]

This will probably involve adding a new if between these two lines: https://github.com/getodk/central-frontend/blob/82438aa97dcd305aac7973736016163a963b95c2/src/util/request.js#L228-L229. The if should check for a request to Backend where the response is a 413 error that is not a Problem.

[matthew-white]

We think that a Frontend request that receives a 413 error from nginx will involve sending a file (or a JSON representation of a file, as with the entities CSV file). Given that, let's use a message similar to the existing error message that mentions files (mixin.request.alert.fileSize).

check for a request to Backend

I think we should show this message for any 413 error response that is not a Backend Problem, including for requests that are not to Backend. In other words, the code change should happen within the last if, not before it.