Repo update usability fixes

[HebaruSan]

Background / motivation

After KSP-CKAN/CKAN-meta#2159, many users opted in to the Kopernicus Bleeding Edge repo to use Kopernicus on recent game versions.

This PR attempts to modernize repo updates somewhat.

Problems

• In the last few days, @R-T-B's server went down and those users saw errors:
  
  • The stack trace is big and scary and doesn't help users
  • There's no obvious way back to the normal app screens. Cancel is available but does nothing; if you're extremely clever, clicking the Manage mods tab works.
  • There's not a lot of info shown about what's going on during a repo update
• CmdLine's IUser implementation always prints \r\n before RaiseProgress and after RaiseMessage (implicitly via WriteLine). This means that if you alternate them, you end up with separate messages combined onto one line or extra blank lines. The existing code has a number of workarounds in place due to this.
• For the last few releases, we've been getting reports of registry corruption at upgrade, where CKAN/registry.json is truncated and can no longer be loaded. I had this happen once myself far too long ago to be able to investigate it (and probably not related to updating CKAN).

Changes

Core:

• Now the repo update code uses IUser.RaiseProgress to announce the start of each step it performs and IUser.RaiseMessage to announce the completion of each step it performs. This way, you can look at the progress bar to see what CKAN is doing right now and the text box to see what has already been finished.
  • 0%–10% - Querying the ETags to see if we need to update at all
  • 10%–90% - Updating each repo, with the percentages split up evenly across this range
  • 90%–100% - Saving the registry
• Now the repo update code calls IUser.RaiseError instead of IUser.RaiseMessage when there's a download error, and it uses Exception.Message instead of Exception.ToString() to omit the stack trace
• Now registry saves are done in a transaction, so truncated files should be a thing of the past, as if anything goes wrong the old file will be preserved

CmdLine:

• Now ConsoleUser internally tracks whether its last output left the cursor at the start of the line or not. This allows it to alternate RaiseProgress and RaiseMessage calls with no missing or extra line breaks. This makes the above Core changes look right for ckan update.
• Now ckan update no longer prints a message at the start, because the Core code will take care of that

ConsoleUI:

• Now updating the repo jumps the user to a progress bar screen that displays the above messages and errors and progress bar updates from Core. Previously it was just a non-interactive dialog box that went away when the update finished.
  (I'm kind of proud how easy it was to do this, just a few lines to add a whole new screen flow.)

GUI:

• Now if a repo update fails, the changes to Core will announce them with an error popup that doesn't have a stack trace. Once the user closes the error, GUI switches back to the mod list and hides the progress bar tab.

[DasSkelett]

Looks good. Let's give this a try, maybe we can prevent some broken registries.