title: Invalid credentials raises exception on auth mutation
body: <p>I'm using graphene-django and django-graphql-jwt in my django project. When I set invalid credentials server raises 'Invalid credentials' exception.</p>
  
  <p>Are invalid credentials supposed to raise an exception on the server? When I test tokenAuth mutation with wrong data Django server raises exception.</p>
  
  <p>Django server log:</p>
  
  <pre><code>  File "/Users/cgf/.local/share/virtualenvs/testdjangoauthbackend-183R1gMP/lib/python3.7/site-packages/promise/promise.py", line 487, in _resolve_from_executor
      executor(resolve, reject)
    File "/Users/cgf/.local/share/virtualenvs/testdjangoauthbackend-183R1gMP/lib/python3.7/site-packages/promise/promise.py", line 754, in executor
      return resolve(f(*args, **kwargs))
    File "/Users/cgf/.local/share/virtualenvs/testdjangoauthbackend-183R1gMP/lib/python3.7/site-packages/graphql/execution/middleware.py", line 75, in make_it_promise
      return next(*args, **kwargs)
    File "/Users/cgf/.local/share/virtualenvs/testdjangoauthbackend-183R1gMP/lib/python3.7/site-packages/graphql_jwt/decorators.py", line 106, in wrapper
      result = f(cls, root, info, **kwargs)
    File "/Users/cgf/.local/share/virtualenvs/testdjangoauthbackend-183R1gMP/lib/python3.7/site-packages/graphql_jwt/decorators.py", line 89, in wrapper
      _('Please, enter valid credentials'))
  graphql.error.located_error.GraphQLLocatedError: Please, enter valid credentials
  </code></pre>