Improve UnableToEmitSwitch warning message

[japgolly]

Previously it would read:

Could not emit switch for @switch annotated match since there are not enough cases

Now it reads:

Could not emit switch for @switch annotated match since a minimum of 4 cases (including the default) are required

[smarter]

The change by itself looks fine but the open PR #9852 should completely get rid of this warning which we agreed was a better solution in #9776

[smarter]

Closed in favor of #9852