stdio: raise StdioError if something goes wrong in ui._write_err The prior code used to ignore certain classes of error, which was not the right thing to do.

stdio: raise StdioError if something goes wrong in ui._write

stdio: catch StdioError in dispatch.run and clean up appropriately We attempt to report what went wrong, and more importantly exit the program with an error code. (The exception we catch is not yet raised anywhere in the code.)