improve failed launch UX

[keturn]

There's lots of room for improvement when it comes to how the launcher behaves when the game fails to launch!

I'm making a list here, but I think incremental improvements are possible and probably want to be broken out in to a series of pull requests, no need to take on the entire list at once.

I'm testing on Linux and in today's case, the error comes from passing a bad parameter in userJavaParameters. The advantage of that is that it's really easy to make up some bad parameters when testing this workflow yourself.

• launcher freezes if game fails to start #551 when the game doesn't start, the entire launcher UI freezes. Hitting the Close button on the window leads to the window manager telling me it's not responsive, and it has to close it forcefully.
• errors from the game process must at least make it in to a log file somewhere.
  Currently, GameRunner does route all output from the game process (both standard output and standard error streams) to the launcher's logs, but it does so at the trace loglevel, which is often dropped entirely.
• the launcher should at least link the user to that file (or the launcher could have an interface to show that log itself).
• after a failed launch, show specifically the error from the failed process, e.g. the Unrecognized VM option 'UseParNewGC'. The user might need the rest of the launcher log to figure out where things went bad, but it also contains a lot of things that will be irrelevant. Isolating the error message is a good starting place.