STM: invalid states are first class citizens

A state in the cache (document node) is now one of "Empty | Error | Valid".
This paves the way to commands/blocks-of-commands resilient-to/confining
errors: one can catch and "ignore" the exception obtained by reaching the
previous state and do something sensible, like running anyway the command
or skipping until the end of an error-confining block is reached.

Invalid states carry an enriched exception with the safe_id attached, so
that if one edits_at or observe them gets a safe place to land (CoqIDE
needs such piece of info).

Little API change in Stm.state_of_id now returning a `Error variant for
the new kind of state.

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