Remove PtraceRead error message.

This error message has no meaning and makes it look like an unwinding
failure occurred. These messages are showing up more often now
that a lot more of debuggerd is using the ReadWord and Read calls.

If a higher level function wants to indicate there is an error, then
it can add a more meaningful error.

Bug: 21818730
(cherry picked from commit 35c8136d)

Change-Id: Ie6e20fbc18702fde935f2cf8cbf1b297fda43282