Fall back to doing a _hardRollback when op.type.invert fails

[hsource]

Motivation

When we had rollbacks due to conflicts on our server, the rollback would fail when the rejected ops were rich-text ops, which aren't invertible due to the error in #442.

This fix is a less breaking change than what's proposed in #442, #443, and #451, and can potentially close those issues. Unlike those changes, this change:

1. Doesn't require any changes in libraries to support it
2. Only changes the behaviour in cases that are totally broken today

Closes #442.

Fix

When the ops fail to invert, just fall back to a hard rollback (refetching the data from the server)

Testing

Added automated test for this case that failed before, but passes now.

[alecgibson]

I think wrapping this in a try/catch is sensible, but I also think this is a bug in json0? I'm right in thinking this doesn't happen if you use a "pure" (non-sub-typed) rich-text document, right? Which is because json0 doesn't check if its subtypes support invert

Edit: on reflection, I'm not sure json0 can do anything particularly useful in this case other than throw, so we should definitely still catch this error, and I'll try to make the json0 error more descriptive.

[coveralls]

Coverage increased (+0.002%) to 97.375% when pulling 20dbec2 on hsource:fix-invert into 46c4aba on share:master.

[alecgibson]

Nice one! Thanks for this.