MatrixRequestError.content should be parsed

Currently the ``content`` is left as a JSON string which means the end application has to parse it, when really it could be done at the api level. Alternatively the ``errcode`` and ``error`` keys could be part of the exception object.

Start by locating MatrixRequestError and the code that constructs it, then inspect how response content, errcode, and error are currently exposed. The issue presents two alternatives, so completion requires deciding whether content should be parsed or the exception should expose errcode and error, with tests confirming the chosen behavior.

Written by the indexing model from the issue text.