target: use LOG_USER to print errors in events

target: use LOG_USER to print errors in events

Events are supposed to be executed asynchronously and not linked
with any command, so it's incorrect printing out an eventual error
using command_print().
Even in the case of events triggered by the command
	<target> invoke-event <event-name>
the eventual error in the event handler is not propagated to the
command, so the command_print() is still not appropriate.

Replace command_print() with LOG_USER() in target_handle_event().
Add further info in the log to identify which event has generated
the error and its associated target.
Before return, clean-up the error message and the stacktrace from
the error already displayed, to avoid propagate them.

Change-Id: I5966aa25e7c81348f4ad4d6e7ce5d4dee44ea4c8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5049
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>