For the record, here's what I ended up committing.  The hex2bin sniffing 
idea is not a winner, because it calls error() if it sees a non-hex 
character, so I just added a prefix character 'X' that signals that the 
error message is hex-encoded instead of plain text; targets can do 
either as they prefer.  The error message is never NULL, I added the MI 
status bit overlooked before, and improved the trace file generator's 
portability, although it could probably use more.  (A bit of a tricky 
program, but a useful investment, as independent implementation of file 
format.)

Stan

2010-03-25  Stan Shebs  <stan@codesourcery.com>

    * tracepoint.h (trace_stop_reason): Add tracepoint_error.
    (struct trace_status): New field error_desc.
    * tracepoint.c (stop_reason_names): Add terror.
    (current_trace_status): Ensure non-NULL error description.
    (trace_status_command): Add error report.
    (trace_status_mi): Ditto.
    (trace_save): Add special case for error description.
    (parse_trace_status): Add case for errors.

    * gdb.texinfo (Tracepoint Packets): Document trace error status.

    * gdb.trace/tfile.c: Generate an additional trace file, improve
    portability.
    * gdb.trace/tfile.exp: Test trace file with an error stop, delete
    files in a better way.