Kill off MatrixCodeMessageException

This code brings the SimpleHttpClient into line with the
MatrixFederationHttpClient by having it raise HttpResponseExceptions when a
request fails (rather than trying to parse for matrix errors and maybe raising
MatrixCodeMessageException).

Then, whenever we were checking for MatrixCodeMessageException and turning them
into SynapseErrors, we now need to check for HttpResponseExceptions and call
to_synapse_error.