Handle constraints on delete

[ringvold]

Currently when deleting certain objects/ecto models an exception is raised if the database constraints is violated. This should be presented to the user as an understandable error message.

Example: When trying to delete a server that is connected to matches or deleting teams that are used in matches, users should be notified about with an error message.

Investigate in which situations the constraints should be ignored/removed. Should you be able to delete a server even if it is connected to many matches? Should it cascade and take the matches with it when deleting?