Make close-{readable,writable} symmetric in taking an optional error-context

[lukewagner]

The 0.3 updates to wasi:http in wasi-http/#143 helped highlight the fact that the reasoning for why we want to be able to pass an error-context into stream.close-readable and get an error-context out of stream.read applies just as well in the opposite direction (to stream.close-writable and stream.write, resp.). This PR basically just removes one of the few asymmetries from the otherwise-quite-symmetric read/write code.