Gitaly has started to return detailed errors in the context of
UserMergeBranch, which was the first proof of concept to see whether
this new model makes sense for error handling. With the introduction of
structured error handling we also had to convert Rails to handle these
new types of errors, which led to the introduction of the new helper
function `#decode_detailed_error`. This function both is responsible for
decoding the error, and translating it into Rails-specific errors.

While it makes sense to have it decode errors, the concrete translation
into Rails-specific errors is something that is specific for every RPC:
UserMergeBranch can return one set of error details, while other RPCs
may return a different set of error details. Mixing this all into a
single function that is supposed to be shared between RPCs thus does not
make much sense.

Refactor the code to move the RPC-specific error translation into the
RPC's own error handling. This prepares for the introduction of error
detail handling in UserSquash.