Return a better error message when nginx returns 413 error

[ktuite]

Closes getodk/central#610

What has been done to verify that this works as intended?

I artificially lowered the nginx error to 1mb to trigger this.

Why is this the best possible solution? Were any other approaches considered?

I didn't see any filled in content-length info that I could have used to show the actual size in the error message. Also left the message pretty generic.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

Does this change require updates to user documentation? If so, please file an issue here and include the link below.

Before submitting this PR, please make sure you have:

• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced