server: Improve error handling for admin accounts

Instead of the assertion we had before, let's throw a WebRobotError
encoding exactly what has failed. This should give a more helpful error
message.

For example now if server.admin.create('xxxxxxxx') is used:

hetzner.WebRobotError: Unable to create admin account:
    Mindestens ein Gro?buchstabe, Mindestens eine Zahl oder ein
    Sonderzeichen

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

hetzner/server.py

@@ -15,7 +15,7 @@
 except ImportError:
     from urllib.parse import urlencode
 
-from hetzner import RobotError
+from hetzner import RobotError, WebRobotError
 from hetzner.rdns import ReverseDNS, ReverseDNSManager
 from hetzner.reset import Reset
 from hetzner.util import addr, scraping
@@ -212,13 +212,26 @@ def create(self, passwd=None):
         }
 
         if not self.exists:
+            failmsg = "Unable to create admin account"
             path = '/server/adminCreate/id/{0}'.format(self._serverid)
         else:
+            failmsg = "Unable to update admin account password"
             path = '/server/adminUpdate'
             data['id'] = self._serverid
 
         response = self._scraper.request(path, data)
-        assert "msgbox_success" in response.read().decode('utf-8')
+        data = response.read().decode('utf-8')
+        if "msgbox_success" not in data:
+            ul_re = re.compile(r'<ul\s+class="error_list">(.*?)</ul>',
+                               re.DOTALL)
+            li_re = re.compile(r'<li>\s*([^<]*?)\s*</li>')
+            ul_match = ul_re.search(data)
+            if ul_match is not None:
+                errors = [error.group(1)
+                          for error in li_re.finditer(ul_match.group(0))]
+                msg = failmsg + ': ' + ', '.join(errors)
+                raise WebRobotError(msg)
+            raise WebRobotError(failmsg)
         self.update_info()
         self.passwd = passwd
         return self.login, self.passwd