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
Change-Id: I83aca9cf241aee99e7425059b1b6e0d229c08a5e