computed.alias gets optional readOnly flag

Aliasing properties is powerful, to some peoples
surprise they alias both set and get. Usually this
is the behaviour one wants but sometimes, as 
https://github.com/emberjs/data/commit/7b88a92c090dddf2d554fb4d4095322f9eef8a59 
suggests, this behaviour is not at all wanted.

When the read only flag is flipped on, a set will
throw and Error. Preventing the set, and providing
useful feedback.