I am trying to create a collection which already exists (a capped one, if this
matters). As far as the method returns `CommandResult`, I expect it to return
an error information (and I write ahead some code that check for `Ok` and uses
the error message). But instead the method throws an exception.

I propose either to document this exception (it is not) or, better, to not
throw but return an error. The latter is more expected and consistent with,
say, `DropCollection` which does not throw on missing collections but returns
error information.