[FIX] web: properly handle failures in onchanges

With the new views, we mainly focused on making them work.  But we also
need to be able to handle unusual cases, such as some RPC failing.  An
example of such a problem was the following:

- a form view with a one2many with an onchange
- edit mode, change a field in the one2many, click elsewhere
- the onchange triggers, calls the server
- it may happen that the server fails with a ValidationError
- the onchange deferred fails, the line in the one2many is restored to
  the value known by the list renderer, which is the previous value (but
  the new value was applied to the model)
- we now have a 'corrupted' state: clicking on save will send the
  invalid value, even though the UI displays the previous one.

We solve this problem in this commit by making sure that the onchange
deferred returned by the _performOnchange always succeed (or stays
pending).  If the rpc fails, the deferred will succeed with an empty
dictionary.  This means that the list renderer will be updated with the
value from the model.

Note that it may not be intuitive that a failure in the onchange is not
seen as an error from the perspective of the _performOnChange, but I
think that it still makes sense:
- the validation/network errors are not really a concern of the UI. The
  UI just display the values entered by the user.
- if an onchange fails (for example, because of a coding error), we do
  not really want to prevent the user from working.

Also, when the onchange is performed, the initial change was already
applied to the local data.  Rollbacking them would be a more difficult
task.

Note that we also improve the mock server in this commit, to allow
mocking failures.

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