btrace: reword error messages

btrace: reword error messages

Reword some btrace error messages to align with the format discussed in
https://sourceware.org/ml/gdb-patches/2018-02/msg00135.html.

gdb/
	* remote.c (remote_btrace_maybe_reopen): Change error message.
	* btrace.c (btrace_enable): Likewise.
	(parse_xml_btrace): Likewise.
	(parse_xml_btrace_conf): Likewise.

testsuite/
	* lib/gdb.exp (skip_btrace_pt_tests): Update expected error message.
	Fix test name.