Print errors that stop the interactive session in plain words

Summary

• When an error stops the interactive session and fx has no message of its own for it, fx now prints plain words instead of the raw error name. For example, fx --resume <id> with a file where the session store's folder belongs prints fx: session path unsafe instead of fx: SessionPathUnsafe.
• A relaunch that fails after an update now names its cause the same way, such as relaunch failed: permission denied.
• The raw error name still goes to the trace log.