On failed message send, show error dialog

[gnprice]

When you hit the send button in the compose box, and the send request fails for whatever reason, we currently drop that error on the floor; we don't show the user any feedback about the failure. We should fix that, by showing an error dialog box.

For examples of how to do this, see the existing call sites of showErrorDialog.

(Historical note: I think the only reason we haven't already done this is that this feature, being so critical to using Zulip, was added early in the prototype before we'd worked out dialog boxes. That's a natural thing to happen, but a bit ironically it means that we've had error handling for a number of less-critical actions for a while without yet having it here.)

This is effectively a simplified version of #720: for this issue, we won't yet make any changes to the compose box and send button, like to give them a "disabled" appearance or keep the text available. Those will be good improvements to make (and #720 tracks making them), but even just showing the error dialog will be an improvement over the status quo.

One effect of this is that until the permission check #791 is implemented, we'll be in much the same position for that feature as zulip-mobile now is (since zulip/zulip-mobile#5878).

Other reports of this issue, indirectly, appear in:

• msglist/dm: Provide proper feedback when sending DMs to a deactivated user #675
• Await send request; handle failure #720