Return an error if WithTargetMessageSize is too large

[Stebalien]

It should be less than 2MiB (ideally less than 1MiB). Messages over 4MiB will be rejected by the remote endpoint per protocol.

The tricky part here is that there's no good way to return an error from this constructor, so this change will require a bit of a refactor.

[BigLep]

2022-10-04 conversation: @guseggert will take a look to determine priority and whether this should occupy attention this month.

[dirkmc]

Note that this is not a high priority for boost work as we found a more effective way to increase parallelization: increasing the worker count, rather than increasing the message size