Improved handling failures when processing remote proposals

Ignore-this: 95cdc277fb2a35f67322bcbbe5b013ff

darcs-hash:20140909124541-56b4d-08ff43eef72e4578f860a1d919f693cc0b8bf77e
Reply with 488 if a remote SDP offer has deleted streams

Ignore-this: cc5c7d87578a37937dd76efb7a319030

darcs-hash:20140909132143-56b4d-eeba1252dc5ddb0df1d3a12f94559edb7831351d
Reuse disabled SDP streams

Ignore-this: 57c489ccff2558e998312cc9a540cc50

darcs-hash:20140909132146-56b4d-12913d835a831b7d9b037300e7928ce56dcfb6af