btrace: improve enable error messages

btrace: improve enable error messages

Improve the error message when GDB fails to start recording branch trace.

This patch also removes a zero buffer size check for PT to align with BTS.  The
buffer size can not be configured to be zero.

gdb/
	* nat/linux-btrace.c (perf_event_pt_event_type): Improve error message.
	Remove parameter and change return type.  Update callers.  Move it.
	(linux_enable_bts, linux_enable_pt): Improve error message.
	(linux_enable_pt): Remove zero buffer size check.
	(linux_enable_btrace): Improve error messages.  Remove NULL return
	check.