Add "error_message+" feature to qSupported

Add a new 'error_message' feature to the qSupported packet. When GDB
supports this feature then gdbserver is able to send
errors in the E.errtext format for the qRcmd and m packets.

Update qRcmd packet and m packets documentation as qRcmd newly
accepts errors in a E.errtext format.
Previously these two packets didn't support E.errtext style errors.

Approved-By: Tom Tromey <tom@tromey.com>
Approved-By: Andrew Burgess <aburgess@redhat.com>