Add XQEMU startup/runtime error handling

[mborgerson]

If XQEMU fails to start, which might be caused by an invalid command line being passed to it (e.g. use controllers which are not connected), XQEMU-Manager will fail to connect to XQEMU over QMP. A cryptic "WinError 10061" error message will be displayed to users. In this case, we should be able to determine why XQEMU failed to start and report this to the user.

Additionally, XQEMU-Manager isn't aware of when the XQEMU subprocess crashes (e.g. hitting an assert), but it should be. We should be able to collect error messages and display them to the users. In the future, automated error reporting would be nice too but that's a separate issue.

[gandalfthewhite19890404]

Xqemu-manager does not handle xqemu stopping either - tested on xqemu 5d4058d6 with xqemu-manager 8e7cfe7.