Handle "timestamp-error" <span> elements

[PIG208]

When a timestamp is created in a broken format (such as <time:asdasd>), the server generates a span element with the timestamp-error class.

We should handle that and display something like "Invalid time format: asdasd" like the server does.

Example

[gnprice]

Pushing to post-launch as this is difficult to trigger accidentally, and empirically rare: 10 examples in your survey results in #921, the latest of them in 2021. (And the consequences are limited when it does happen: the one affected message shows one form of error instead of a different form of error.)