No default text/plain formatters for builtin errors

[jrmithdobbs]

Accept: text/plain.*

Does not throw useful errors for 404 etc.

text/html and application/json have these default handlers.

Is there simple boilerplate that could be added to work around this for an app using json and plaintext formats?

[gregwebs]

Looks like text/plain is missing here:
https://github.com/yesodweb/yesod/blob/master/yesod-core/Yesod/Core/Class/Yesod.hs#L422

Give a shot at adding it if you feel like. It seems that what is in the JSON probably works as plain text, but we would lead with what is in the 'message` key.

[StevenXL]

Addressed in #1520.