Throw DOMExceptions where appropriate (fixes #2518).
auto merge of #2519 : Ms2ger/servo/exn, r=jdm
auto merge of #2515 : saneyuki/servo/raw, r=jdm

Fix #2511

@jdm r?
Remove needless '&mut self' from HTMLStyleElementMethods.
Remove needless '&mut self' from HTMLMapElementMethods.
Remove needless '&mut self' from HTMLMediaElementMethods.
Remove needless '&mut self' from HTMLMetaElementMethods.
Remove needless '&mut self' from HTMLMeterElementMethods.
Remove needless '&mut self' from HTMLModElementMethods.
Remove needless '&mut self' from htmlobjectelement.rs.
Remove needless '&mut self' from HTMLOListElementMethods.
Remove needless '&mut self' from HTMLOptGroupElementMethods.
Remove needless '&mut self' from HTMLOptionElementMethods.
Remove needless '&mut self' from HTMLOutputElementMethods.
Remove needless '&mut self' from HTMLParagraphElementMethods.
Remove needless '&mut self' from HTMLParamElementMethods.
Remove needless '&mut self' from HTMLPreElementMethods.
Remove needless '&mut self' from HTMLProgressElementMethods.
Remove needless '&mut self' from HTMLScriptElementMethods.
Remove needless '&mut self' from HTMLSelectElementMethods.