glamr/accounts: fix AddUserForm input validation

Raise appropriate exception when email input field is empty.

mibios/glamr/accounts.py

@@ -40,7 +40,7 @@ def clean_emails(self):
         data = self.cleaned_data['emails']
         data = data.split()
         if not data:
-            raise RuntimeError('BORK empty')
+            raise ValidationError('input field is empty')
 
         errors = []
         for item in data: