MI error codes

[Vladimir Prus]

I've just this MI error message:

	34^error,msg="Warning:\n
	Could not insert hardware watchpoint 7.\n
	Could not insert hardware breakpoints:\n
	You may have requested too many hardware breakpoints/watchpoints.\n"

The natual reaction of a GUI to this message will be to show user the breakpoint
widget. Unfortunately, this is only possible by looking at the 'msg' string, which is
not specified in any way.

Can you add some 'error_code' field that will have strictly defined values that UI
can use to perform handling specific to each kind of error?

- Volodya