New struct. (backtrace_command_stub): New function. (backtrace_command):
        Call backtrace_command_stub wrapped in a catch_errors call.
        (backtrace_full_command): Ditto.
    New prototypes.
    New functions. (error_stream_1): Ditto. (error_stream): Call
        error_stream_1.
    Replacing 6.1post patch #78.