[18.01] Fix exception if email is not specified or email is wrong

[mvdbeek]

We render a regular error a few lines lower.

The traceback is:

AttributeError: 'NoneType' object has no attribute 'split'
  File "galaxy/web/framework/middleware/sentry.py", line 43, in __call__
    iterable = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/recursive.py", line 85, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/middleware/statsd.py", line 35, in __call__
    req = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python2.7/site-packages/paste/httpexceptions.py", line 640, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 136, in __call__
    return self.handle_request(environ, start_response)
  File "galaxy/web/framework/base.py", line 215, in handle_request
    body = method(trans, **kwargs)
  File "galaxy/webapps/galaxy/controllers/user.py", line 928, in activate
    email = unquote(params.get('email', None))
  File "python2.7/urllib.py", line 1238, in unquote
    bits = s.split('%')