API v2 fixes

[sharkykh]

Better stack trace for API v2 on Python 2

The test:
raise Exception('test') in medusa/server/api/v2/log.py @ LogHandler # get
Before:

Traceback (most recent call last):
  File "c:\Dev\Medusa\ext\tornado\web.py", line 1592, in _execute
    result = yield result
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1133, in run
    value = future.result()
  File "c:\Dev\Medusa\ext\tornado\concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1141, in run
    yielded = self.gen.throw(*exc_info)
  File "c:\Dev\Medusa\medusa\server\api\v2\base.py", line 99, in async_call
    content = yield IOLoop.current().run_in_executor(executor, prepared)
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1133, in run
    value = future.result()
  File "c:\Dev\Medusa\ext\tornado\concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "<string>", line 3, in raise_exc_info
Exception: test

After:

Traceback (most recent call last):
  File "c:\Dev\Medusa\ext\tornado\web.py", line 1592, in _execute
    result = yield result
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1133, in run
    value = future.result()
  File "c:\Dev\Medusa\ext\tornado\concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1141, in run
    yielded = self.gen.throw(*exc_info)
  File "c:\Dev\Medusa\medusa\server\api\v2\base.py", line 80, in async_call
    content = yield tornado_future
  File "c:\Dev\Medusa\ext\tornado\gen.py", line 1133, in run
    value = future.result()
  File "c:\Dev\Medusa\ext\tornado\concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "c:\Dev\Medusa\medusa\server\api\v2\base.py", line 71, in wrapper
    result = method(*args, **kwargs)
  File "c:\Dev\Medusa\medusa\server\api\v2\log.py", line 93, in get
    raise Exception('test')
Exception: test

Fix text/plain response charset:

Before:
2019-07-10 02:12:30 INFO MAIN :: [f1dc11a] 76703: Unable to find IMDb info in the database: Pokֳ©mon
After:
2019-07-10 02:13:23 INFO MAIN :: [f1dc11a] 76703: Unable to find IMDb info in the database: Pokémon