Fix #171: UltiSnips crashes in new file with text containing 0x80 char

Here is a fix for #171 with a test.  @SirVer I hope you don't mind, but I added an extra line into the `vim_config` variable to reset the unnamed register, since it could be carried across sessions.  I found that when I had my test in a failing state, it would induce a failure across several tests because the unnamed register was being persisted.  I figured that resetting it would make the test harness a little more robust against that kind of failure.  Also, since snippet expansion would still complete--even with the traceback being emitted--I had to examine messages for the error.  I hope it's robust enough for your liking.  As an added bonus, if I detect an error, I paste the messages into the buffer, so you'll see the traceback in the failing test output.  I hate to say this, but putting together a test for this took *considerably* longer than expected. :-(