runtime: improve message when a bad pointer is found on the stack

Currently this message says "invalid stack pointer", which could be
interpreted as the value of SP being invalid. Change it to "invalid
pointer found on stack" to emphasize that it's a pointer on the stack
that's invalid.

Updates #16948.

Change-Id: I753624f8cc7e08cf13d3ea5d9c790cc4af9fa372
Reviewed-on: https://go-review.googlesource.com/28430
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/runtime/stack.go

@@ -598,7 +598,7 @@ func adjustpointers(scanp unsafe.Pointer, cbv *bitvector, adjinfo *adjustinfo, f
 				// Live analysis wrong?
 				getg().m.traceback = 2
 				print("runtime: bad pointer in frame ", funcname(f), " at ", pp, ": ", hex(p), "\n")
-				throw("invalid stack pointer")
+				throw("invalid pointer found on stack")
 			}
 			if minp <= p && p < maxp {
 				if stackDebug >= 3 {