Arvados

This is the same message seen in #4776, so addressing this will likely help that along.

• Default fall-back error message should be more friendly than "error". Perhaps: "Sorry, I don't have any further details about what the error was."
• Try some other things before the fall-back. If there's an exception, a stack trace might be accessible (catch(e){var foo=e.stack}), and we can show the whole thing or part of it.
• Show a "report an issue" button if the error came with an HTTP status code.
• See if we can get a useful error message from the browser in the cases that are known to be common, like timeout.