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.)

stdio: add machinery to identify failed stdout/stderr writes Mercurial currently fails to notice failures to write to stdout or stderr. A correctly functioning command line tool should detect this and exit with an error code. To achieve this, we need a little extra plumbing, which we start adding here.