Fixes silent failure issues

[jmuharsky]

In cases where error messages aren't returning a string, Explorer is silently failing, and the widget will continue to appear in a loading state. This fixes the issue so that "An unexpected error occurred" appears if no error text is provided.

[jmuharsky]

Demo is at https://27-1-dot-perfkit-explorer.googleplex.com

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[jmuharsky]

Cleaned up comments. PTAL?

[jmuharsky]

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