Add a "try again later" message for transient errors

Merged Nate Graham requested to merge ngraham/knewstuff:tell-the-user-to-try-again-later into master

This error dialog currently detects transient errors and changes the icon accordingly. But it doesn't add any text. As a result, the fact that the error is transient and the user should just try again later remains unclear.

This commit fixes that by appending some text to indicate it as well.

@leinir @alex @teams/usability

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