Document "E.MESSAGE" RSP errors

Document "E.MESSAGE" RSP errors

For many years, GDB has accepted a "E.MESSAGE" error reponse, in
addition to "E NN".  For many packets, GDB strips the "E." before
giving the error message to the user.  For others, GDB does not strip
the "E.", but still understands that it is an error, as it starts with
"E", and either prints the whole string, or ignores it and just
mentions an error occured (same as for "E NN").

This has been the case for as long as I remember.  Now that I check, I
see that it's been there since 2006 (commit a76d924dffcb, also here:
https://sourceware.org/pipermail/gdb-patches/2006-September/047286.html).
All along, I actually thought it was documented.  Turns out it wasn't.

This commit documents it, in the new "Standard Replies" section, near
where we document "E NN".

The original version of this 3-patch documentation series was a single
CodeSourcery patch that documented the textual error as
"E.NAME.MESSAGE", with MESSAGE being 8-bit binary encoded.  But I
think the ship has sailed for that.  GDBserver has been sending error
messages with more than one "." for a long while, and with no binary
encoding.  Still, I've preserved the "Co-Authored-By" list of the
original larger patch.

The 'qRcmd' and 'm' commands are exceptions and do not accept this
reply format.  The top of the "Standard Replies" section already says:

  "All commands support these, except as noted in the individual
  command descriptions."

So this adds a note to the description of 'qRcmd' and 'm', explicitly
stating that they do not support this error reply format.

Change-Id: Ie4fee3d00d82ede39e439bf162e8cb7485532fd8
Co-Authored-By: Jim Blandy <jimb@codesourcery.com>
Co-Authored-By: Mike Wrighton <mike_wrighton@mentor.com>
Co-Authored-By: Nathan Sidwell <nathan@codesourcery.com>
Co-Authored-By: Hafiz Abid Qadeer <abidh@codesourcery.com>
Approved-By: Eli Zaretskii <eliz@gnu.org>