Consider making error codes spec-independent (put them in the appendix)

Many are reusable outside of the spec they are defined in, such as `M_BAD_JSON` and `M_FORBIDDEN`. Some are very tightly scoped, like `M_THREEPID_DENIED`, but may still be of use to define outside of one particular area.

Should build off of the suggestions in https://github.com/matrix-org/matrix-doc/issues/603.

Start by reviewing the suggestions in matrix-org/matrix-doc#603 and the current error-code definitions in the Matrix specification. Determine which codes can be made spec-independent and how they should be organized in an appendix; the work is done when the proposed organization and affected definitions are consistently updated.

Written by the indexing model from the issue text.