remote.c: Use packet_check_result

when processing the GDBserver reply to qRcmd packet.
Print error message or the error code.
Currently, when qRcmd request returns an error,
GDB just prints:

Protocol error with Rcmd

After this change, GDB will also print the error code:

Protocol error with Rcmd: 01.

Add an accept_msg argument to packet_check result. qRcmd
request (such as many other packets) does not recognise
"E.msg" form as an error right now. We want to recognise
"E.msg" as an error response only for the packets where
it's documented.

Also use packet_check result in remote_read_bytes_1.

Approved-By: Andrew Burgess <aburgess@redhat.com>