don't assume ucm trunk versions start with latest-

[aryairani]

closes #3233

UCM versions are (intentionally) based on git describe and hence sensitive to the git repo state. When the welcome flow was written, trunk UCM builds tended to have a version number that began with latest-, but that won't necessarily be the case, and isn't the case today.

The result was that Main.launch was hitting this DontDownloadBase case instead of complaining about the unknown library:

unison/unison-cli/unison/Main.hs

Lines 391 to 393 in 265157b

	let downloadBase = case defaultBaseLib of
	Just remoteNS | shouldDownloadBase == ShouldDownloadBase -> Welcome.DownloadBase remoteNS
	_ -> Welcome.DontDownloadBase

Controversial decisions

Maybe line 393 should be an error?