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Catch synchronous errors in `Doc._otApply`

Fixes https://github.com/share/sharedb/issues/257

As outlined in the above issue, calling `Doc.submitOp` can sometimes
`throw` a synchronous error, for example when submitting an invalid op
that causes `type.apply` to `throw`.

This is surprising behaviour, because an error handler should already be
provided in the callback supplied to `submitOp`. What's more, the doc
could be left partially mutated and out-of-sync with the server, which
could lead to confusing behaviour.

This change wraps all internal usage of `Doc._otApply` in `try`/`catch`
blocks. If an error is thrown in this method, then we attempt to
recover with `Doc._hardRollback`, which will attempt to reset the
document and then call the callbacks of the pending ops with the error.

This change also prevents `Doc._hardRollback` from emitting an error if
at least one pending op callback was invoked, so that we don't branch
error handling behaviour.

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