Remove orphaned GenericError class (#10500)
Originally introduced in
https://github.com/matrix-org/matrix-react-sdk/pull/8329
All usages were removed in
https://github.com/matrix-org/matrix-react-sdk/pull/9605
This error also has problems with showing tranlsated versions in the logs.
Part of https://github.com/vector-im/element-web/issues/9597
We should be using the new `UserFriendlyError` for this kind of thing anyway.
Introduced in https://github.com/matrix-org/matrix-react-sdk/pull/10440
