Don't throw an error in "show mpx bound" implementation

Don't throw an error in "show mpx bound" implementation

"show" functions should not throw an exception in part because it causes
the output of the commands "info set" and "show" to get truncated.

This fixes the following fails:

    FAIL: gdb.base/default.exp: info set
    FAIL: gdb.base/default.exp: show

gdb/ChangeLog:

	* i386-tdep.c (i386_mpx_info_bounds): Don't call error, instead
	use printf_unfiltered.
	(set_mpx_cmd): Add missing trailing space to command string
	literal.
	(_initialize_i386_tdep): Give the "mpx" prefix command its
	correct name.